A、Their memory. B、The stars. C、The sense of time. D、Their experience. B此题四个选项中A、B、C均在文中被提及，但与birds flying at night有关的只有star,因此B为

How do the birds manage to find their way? There seems to be no single answer: (32)they use many methods. Some we are beginning to understand;  and there may be some that depend on abilities we have not yet suspected. (33) Many birds certainly follow major geographical features. Migrants follow recognized land, fly over familiar sea and arrive at their summer homes.
    But all birds cannot use such straightforward methods. An Arctic sea bird, for example, has to fly at least 3,000 kilometers across the Antarctic Ocean with no land to guide it. We know that some birds flying at night,  (34) navigate by the star for on cloudy nights they tend to get lost.
    Day-flying birds may use the sun. If they are to do so, they must (35) have a precise sense of time. Still others appear to be able to use the earth’s magnetic field as a guide. So it seems that many migrating birds must carry in their brains a clock, a compass and the memory of a map.