单选题：根据下列材料，回答题。　　In September, in Britain, you may see a lot of

根据下列材料，回答题。
　　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.
第36题答案为（　　）。 A．being perched
B．perching
C．being perching
D．be perched