TOEFL Reading: ETS-TOEFL阅读机经 - 357DS9VP23X10H14L$

Which of the following facts demonstrate that migrating birds need a "timing mechanism?" A. Sometimes a migrating bird's internal compass does not function correctly. B. The Sun is visible to migrating birds in the east in the morning, but in the west in the evening. C. Most birds migrate either during the day or during the night, but not both. D. Some stars, for example the North Star, do not appear to change position during a single night or even during the course of a long migration.