题目内容：
根据以下材料回答题In September, in Britain, you may see a lot of birds 1～on roofs and telegraph wires. These birds are swallows. They are___2___together because, very soon, they will be flying___3___to much warmer lands, where they will find ___4___small flying insects on which they _5__ .There are no such insects_6__in Britain during the winter, as it is _7__ Cold for them.
The swallows settle, fly off, swoop, and ___8__again.This they do many times, for they are making short__9__flights in order to be fit for the long joumey __10__them.
_11_of these migrating birds leave Britain in the autumn. They fly__12__for hundreds of miles__13__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 __14__ ．
In the spring of the following year they __15__ the long and tiring journey back to Britain. They return to the identical barn or tree in the__16__district which they had left the___17___autumn. How do these birds find their___18___there and back over such vast distances? Nobody knows exactly___19___，but it has something to do____20___winds and air currents.
A．being perched
B．perching
C．being perching
D．be perched
参考答案：
答案解析：