It is said of parasitic forms of life that, although they burden their hosts, they do not kill them, since...

GMAT Critical Reasoning : (CR) Questions

Source: Mock

Critical Reasoning

Assumption

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It is said of parasitic forms of life that, although they burden their hosts, they do not kill them, since a parasite cannot survive unless its host does. Mr. Craig's prize-winning lilies, however, were invaded by dodder, a parasitic plant, and every one of the lilies died soon after. Plainly, therefore, a parasite can be deadly.

The argument above depends on which of the following assumptions?

Mr. Craig did nothing to control the dodder that was invading his lilies.

Mr. Craig's lilies were not infected with a virus deadly to lilies before being invaded by dodder.

Dodder is not usually a parasite of lilies

Lilies are especially susceptible to being weakened by invading parasites.

Mr. Craig's lilies were invaded by an unusually vigorous strain of dodder.

Solution

Passage Analysis:

Text from Passage	Analysis
It is said of parasitic forms of life that, although they burden their hosts, they do not kill them, since a parasite cannot survive unless its host does.	What it says: There's a general rule that parasites don't kill their hosts because parasites need living hosts to survive What it does: Sets up the conventional wisdom about how parasites behave What it is: Commonly accepted principle
Mr. Craig's prize-winning lilies, however, were invaded by dodder, a parasitic plant, and every one of the lilies died soon after.	What it says: A specific case where dodder (a parasite) invaded lilies and all the lilies died What it does: Presents a real-world example that seems to contradict the general rule we just heard What it is: Factual observation/evidence Visualization: General rule says: \(\mathrm{Parasite} + \mathrm{Host} = \mathrm{Host\ survives}\) Reality with lilies: \(\mathrm{Dodder} + \mathrm{Lilies} = \mathrm{All\ lilies\ died}\)
Plainly, therefore, a parasite can be deadly.	What it says: The author concludes that parasites can actually kill their hosts What it does: Draws a conclusion that directly contradicts the opening principle based on the lily example What it is: Author's main conclusion

Argument Flow:

The argument starts with a widely accepted principle about parasites, then presents a specific counterexample involving lilies and dodder, and concludes that the general principle is wrong.

Main Conclusion:

A parasite can be deadly (can kill its host).

Logical Structure:

The author uses one specific case (lilies dying after dodder invasion) to challenge and overturn a general biological principle. The logic assumes that the lily deaths were actually caused by the dodder parasite, rather than by some other factor.

Prethinking:

Question type:

Assumption - We need to find what the author must believe to be true for their conclusion to logically follow from the evidence

Precision of Claims

The conclusion makes a broad claim about parasites in general ('a parasite can be deadly') based on one specific example (dodder killing lilies)

Strategy

To find the assumption, we need to identify what could falsify the conclusion while respecting the facts. The author sees lilies died after dodder invasion and concludes parasites can be deadly. But what if something else killed the lilies? What if dodder isn't actually a parasite? What if the lilies were already dying? We need to find what the author must assume to make this logical leap from one example to a general conclusion about parasites.

Answer Choices Explained

Mr. Craig did nothing to control the dodder that was invading his lilies.

This doesn't need to be true for the argument to work. Even if Mr. Craig tried to control the dodder but failed, the lilies still died after the dodder invasion, and the author's conclusion that parasites can be deadly would still hold. Whether or not Mr. Craig took action doesn't affect the logical connection between dodder presence and lily death.

Mr. Craig's lilies were not infected with a virus deadly to lilies before being invaded by dodder.

This must be true for the argument to work. The author concludes that the dodder parasite killed the lilies, but if the lilies were already infected with a deadly virus before the dodder arrived, then the virus could have been the real killer, not the dodder. If we negate this assumption (the lilies WERE infected with a deadly virus first), the author's entire conclusion collapses because we can't attribute the deaths to the parasite anymore.

Dodder is not usually a parasite of lilies

Whether dodder commonly parasitizes lilies or not doesn't matter for this argument. Even if dodder regularly attacks lilies, the author's point that this particular case shows parasites can be deadly still stands. The frequency of the relationship doesn't affect the logical conclusion.

Lilies are especially susceptible to being weakened by invading parasites.

This isn't necessary for the argument. The author's conclusion is that parasites CAN be deadly, not that they're only deadly to particularly susceptible hosts. Even if lilies weren't especially susceptible, the fact that they died after dodder invasion would still support the conclusion that parasites can kill.

Mr. Craig's lilies were invaded by an unusually vigorous strain of dodder.

This assumption isn't required. The author's conclusion is broad - that parasites can be deadly. It doesn't matter if this was a normal strain or an unusual one; either way, a parasite killed the host, which proves the general principle wrong and supports the author's conclusion.

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