Passage Summary: If a baby caterpillar's first meal is a nightshade plant, it refuses to eat anything else for the rest of its life. Scientists think this is because a specific chemical in nightshades changes the caterpillar's taste buds so that only that chemical tastes good.

Conclusion: Habituation to the chemical indioside D in nightshades causes tobacco hornworms to lose interest in other food sources.

Reasoning: Hornworms that eat nightshades first refuse other plants, whereas those that start with other plants remain flexible in their diet.

Analysis: The argument relies on a specific chemical, indioside D, being the 'smoking gun' for this behavior. To strengthen this hypothesis, we need evidence that directly links this chemical to the caterpillars' pickiness. Look for an answer that shows what happens when indioside D is isolated—for instance, if caterpillars become picky even without eating a nightshade, as long as they are exposed to that specific chemical. Strengthening the causal link between the chemical and the behavior is the priority here.