TOEFL Reading: ETS-TOEFL阅读机经 - S3D1PJC6F1W551P3L$

In paragraph 1, why does the author discuss the example of a turkey vulture in flight A. To explain why birds sink unless they find currents of rising air B. To provide information about sinking and soaring speeds typical for large birds C. To illustrate why gliding birds seek out rapidly rising air currents D. To clarify the relationship between sinking speed and the speed of rising air needed to soar