OG7 阅读 Q22

The main purpose of the passage is conveyed by the first sentence: "Anyone watching the autumn sky knows that migrating birds fly in a V formation, but scientists have long debated why." The first paragraph continues by focusing on new research that might answer the question of why birds fly in that formation ("presumably to catch the preceding bird's updraft — and save energy during flight"). As a whole, the passage can therefore be seen as a discussion of the biological motivation behind migrating birds' reliance on the V formation.
Choice A is incorrect because the squadrons of planes mentioned in the second paragraph are used as an example to discuss migrating birds but are not themselves the main subject of this passage. Choice B is incorrect because although the fourth paragraph does discuss the role of downdrafts in V-formation flight, this discussion is brief and does not constitute a main purpose. Choice D is incorrect because the passage does not illustrate how birds sense air currents through their feathers; instead, the seventh paragraph suggests in passing that such sensation may play a role in maintaining the V formation: "Scientists do not know how the birds find that aerodynamic sweet spot, but they suspect that the animals align themselves either by sight or by sensing air currents through their feathers."