In September, in Britain, you may see a lot of birds (36) on roofs and telegraph wires. These birds are swallows. They are (37) together because, very soon, they will be flying (38) to much warmer lands, where they will find (39) small flying insects on which they (40) . There are no such insects (41) in Britain during the winter, as it is (42) cold for them. The swallows settle, fly off, swoop, and (43) again. This they do many times, for they are making short (44) flights in order to be fit for the long journey (45) them. (46) of these migrating birds leave Britain in the autumn. They fly (47) for hundreds of miles (48) they reach the warm lands of Africa. But not all the birds get there, for many of them perish in the stormy weather they meet with (49) . In the spring of the following year they (50) the long and tiring journey back to Britain. They return to the identical barn or tree in the (51) district which they had left the (52) autumn. How do these birds find their (53) there and back over such vast distances Nobody knows exactly (54) , but it has something to do (55) winds and air currents. A．whyB．whenC．howD．what