Clarification

Exactly! Great work.

Why is C correct - how do we know there are no birds other than yellow warblers that compete with yellow warblers for food?

Is it because the conclusion says yellow warblers have no competition for the food supply?

Hey @sharpen7, thanks for your post. You may have misread this Strengthen With Necessary Premise question as a Must Be True question. We do not know there are no birds other than yellow warblers competing with yellow warblers for food - but that is the unstated assumption on which the argument in the stimulus relies. Without this unstated assumption, the stimulus's conclusion - that yellow warblers "have no competition for the food supply within the range of their restricted flying" - would not hold. Thus, (C) is the necessary premise needed to strengthen the argument. And this is why it is the correct answer. Hope this helps.