Support Caffeine can kill or inhibit the growth of the larvae of several species of insects. ███ ██████ ██████████ ██████ ████ ███████ ████████ ██████ ███ ████ ████ ██████ █ ███████████ ████ █████████ ██ █████ ██ ██████ ████████ ███ ███████ █████ ███████ █████████ ████ ██████ ██ ████████ ███ ███ ██████████ ████ ███ ████████ ██ ██████████████ ██████████ ██ ████████ ██ ███████ █████ ██ ████ ███████ ███████ ██ ██████ ██ ███ ██████████ ███ ███████ ██ █ ███████ ███ █████ ███████
The argument proposes a hypothesis that plants with a meaningful caffeine concentration evolved caffeine as a defense mechanism, not accidentally. We also learn some evidence that supports this hypothesis: caffeine can kill or harm various insect larvae. The hypothesis is further supported by an experiment showing that the caffeine in tea leaves can kill tobacco hornworm larvae.
This argument follows a phenomenon-hypothesis pattern of reasoning. It attempts to explain an observed phenomenon (caffeine's harmful impact on larvae) with a hypothesis (caffeine as a defense mechanism).
For any phenomenon-hypothesis argument, there's at least one necessary assumption: that the evidence isn't better explained by an alternative hypothesis. It might also be necessary to assume that there's no contradictory evidence out there that defeats the hypothesis.
The details of these kinds of assumptions can be difficult to predict in advance. We can rely on process of elimination, making use of the negation and must-be-true tests to determine which answer choices are unnecessary.
The argument assumes that
caffeine-producing plants are ██ █████████ ███ ████████ ██ ███ ███████████ ██ ██████████ ████████████
(A) is compatible with the argument, but it isn't necessary for the argument to make sense. Even if caffeine-producing plants aren't used for commercial insecticides, it could still be true that those plants evolved caffeine as a defense.
caffeine is stored ██ ██████ ███ █████ █████ ██ ██████████████████ ██████ ██ ██████████████ ███████ █████ ██ ███ ████████ █████████████ ██ ███ ███████████ ███ ██ ███ ███████ ████████ ██████
(B) is a trap answer choice: it tries to trick us into assuming that the only alternative is plants having insufficient caffeine to threaten harmful insects. But that's not true—(B) would also be false if plants had higher caffeine concentrations, which would leave the argument unharmed.
Even if (B) said "equal or higher" concentrations, keep in mind that we don't know how much caffeine is necessary to kill insects. Perhaps the experiment used much more than necessary, and a lower concentration would still kill insects. That's another reason (B) is not necessary to assume.
caffeine-producing plants grow ████████ ██████ ██████ ████ █ █████ ██████ ██ ██████████ ██████ ██ ████ █████ █ █████ ██████ ██ ███ █████████ ██ █████ ██████
(C) seems promising: to evolve caffeine as a defense, there has to be something to defend against. But reading closely, (C) is too strong to be necessary. It makes insects a sufficient condition for plants to evolve caffeine, but really, the argument needs insects to be a necessary condition.
We can see this by negating (C), which would tell us that caffeine-producing plants grow in only some places where insects pose or posed a threat. That's still totally compatible with the argument. Even if some plants didn't evolve caffeine, those that did could have still evolved it as a defense.
the tobacco plant ██ █████ ███ █████ ███████ ████ ███████ ████████ ███ █████ ███ ███████
The argument never mentions the properties of the tobacco plant—and it's not necessary to make assumptions about this specific plant for the argument to make sense in general. We don't even know if tobacco hornworms actually eat tobacco. (D) could be false, and the argument would be fine.
caffeine-producing plants or █████ █████████ ████ ██ ████ ████ ████ ███████ ██ █████ ███ ████ ██ █████████ █████████ ██ ████████
(E) is the better version of (C): it tells us that harmful insects are a necessary condition for caffeine production. If this wasn't the case, then the argument would stop making sense. How could plants evolve caffeine to defend against insects that never threatened them?
If (E) were false, that would effectively support the alternative explanation that plants evolved caffeine at random. It would be ridiculous to say caffeine responded to an evolutionary pressure that wasn't present.
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