The Earth's rivers constantly carry dissolved salts into its oceans. Clearly, therefore, by taking the resulting increase in salt levels...

GMAT Critical Reasoning : (CR) Questions

Source: Official Guide

Critical Reasoning

Assumption

HARD

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The Earth's rivers constantly carry dissolved salts into its oceans. Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the ocean to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimated.

Which of the following is the assumption on which the argument depends?

The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been unusually large during the past hundred years.

At any given time, all the Earth's rivers have about the same salt levels.

There are salts that leach into the Earth's oceans directly from the ocean floor.

There is no method superior to that based on salt levels for estimating the maximum age of the Earth's oceans.

None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans.

Solution

Passage Analysis:

Text from Passage	Analysis
The Earth's rivers constantly carry dissolved salts into its oceans.	What it says: Rivers continuously transport dissolved salts to the oceans What it does: Sets up a basic scientific fact about how salt gets into oceans What it is: Scientific premise Visualization: Rivers → Oceans (salt flowing constantly)
Clearly, therefore, by taking the resulting increase in salt levels in the oceans over the past hundred years and then determining how many centuries of such increases it would have taken the ocean to reach current salt levels from a hypothetical initial salt-free state, the maximum age of the Earth's oceans can be accurately estimated.	What it says: We can calculate ocean age by measuring recent salt increases and extrapolating backward to when oceans had no salt What it does: Draws a conclusion from the salt transport fact, proposing a method to estimate ocean age What it is: Author's main conclusion Visualization: Salt-free oceans (0%) → Current salt levels (100%) = Ocean age calculation method

Argument Flow:

The argument starts with a basic scientific fact about rivers carrying salt to oceans, then jumps to conclude that we can use recent salt level changes to calculate how old the oceans are by working backward to when they were salt-free.

Main Conclusion:

The maximum age of Earth's oceans can be accurately estimated by measuring recent salt increases and calculating how long it would take to reach current salt levels from zero.

Logical Structure:

The argument uses the premise that rivers constantly add salt to oceans to support the conclusion that we can reverse-calculate ocean age from recent salt data. However, this logic assumes the rate of salt increase has been constant over time, which is the key assumption the argument depends on.

Prethinking:

Question type:

Assumption - We need to find what the argument must assume to be true for the conclusion to work. The argument claims we can accurately estimate ocean age by measuring recent salt increases and extrapolating backward.

Precision of Claims

The argument makes a precise quantitative claim about 'accurately estimating' ocean age using a specific calculation method based on salt level increases over the past hundred years.

Strategy

To find assumptions, we need to identify ways this conclusion could fall apart while keeping the facts intact. The facts are: rivers do carry salt to oceans, and we can measure recent salt increases. But what must be true for this backward extrapolation method to work accurately? We should look for conditions about consistency, other factors, and the validity of the calculation approach.

Answer Choices Explained

The quantities of dissolved salts deposited by rivers in the Earth's oceans have not been unusually large during the past hundred years.

This choice identifies a critical assumption the argument depends on. The argument proposes using salt level increases from the past hundred years as a baseline to calculate ocean age by extrapolating backward. If the quantities of salt deposited during this measurement period were unusually large (or unusually small), then using this data to estimate how long it took oceans to reach current salt levels would produce wildly inaccurate results. The argument must assume that the past hundred years represent a typical rate of salt deposition for the calculation to work accurately. This is what the argument must assume to be true.

At any given time, all the Earth's rivers have about the same salt levels.

This choice discusses salt levels in different rivers at the same time, but the argument doesn't depend on rivers having similar salt levels. The argument is about the total amount of salt being added to oceans from all rivers combined over time. Whether individual rivers have different salt concentrations doesn't affect the validity of measuring overall ocean salt increases and extrapolating backward. The argument can work even if rivers have varying salt levels.

There are salts that leach into the Earth's oceans directly from the ocean floor.

This choice actually works against the argument rather than supporting it as an assumption. If salts leach directly from the ocean floor into oceans, this would mean that rivers aren't the only source of salt in oceans. This would make the river-based calculation method less accurate, not more accurate. The argument would need to assume the opposite - that rivers are the primary or only significant source of salt.

There is no method superior to that based on salt levels for estimating the maximum age of the Earth's oceans.

This choice is about whether there are superior methods for estimating ocean age, but the argument doesn't need to assume its method is the best available. The argument only claims that its salt-based method 'can accurately estimate' ocean age - it doesn't claim to be superior to other methods. Whether better methods exist doesn't affect whether this particular method works.

None of the salts carried into the Earth's oceans by rivers are used up by biological activity in the oceans.

This choice addresses what happens to salt after it reaches the oceans, but the argument doesn't depend on this assumption. Even if biological activity uses up some salt, the argument is based on measuring actual increases in ocean salt levels over the past hundred years. These measurements would already account for any salt consumption by biological activity, so the calculation would still be valid.

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