题目内容：
In September，in Britain，you may see a lot of birds36on roofs and telegraph wires.These birds are swallows.They are37 together because，very soon，they will be flying38 to much warmer lands，where they will find39small flying insects on which they40.There are no such insects41 in Britain during the winter，as it is 42cold for them. The swallows settle，fly off，swoop，and43again.This they do many times，for they are making short44flights in order to be fit for the long joumey45 them. 46of these migrating birds leave Britain in the autumn.They fly47for hundreds of miles48 they reach the warm lands of Africa.But not all the birds get there，for many ofthem perish in the stormy weather they meet with49. In the spring of the following year they50 the long and tiring journey back to Britain. They return to the identical bam or tree in the51district which they had left the52autumn. How do these birds find their53 there and back over such vast distances?Nobody knows exactly54，but it has something to do55winds and air currents.
阅读以上短文,回答题.
第36题应填__________ A．being perched
B．perching
C．being perching
D．be perched
参考答案：
答案解析：