题目内容：
回答题In September, in Britain, you may see a lot of birds 36__________ on roofs and telegraphwires. These birds are swallows. They are 37__________ together because, very soon, they will beflying 38 __________ to much warmer lands, where they will find 39__________ small flying insects onwhich they 40__________ There are no such insects 41__________ in Britain during the winter, as it is42__________ cold for them.
The swallows settle, fly off, swoop, and 43 __________ again. This they do many times, for they
are malting 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 ofmiles 48 __________ they reach the warm lands of Africa. But not all the birds get there, for many ofthem 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 ex-actly 54__________ , but it has something to do 55__________ winds and air currents.
A．being perched
B．perching
C．being perching
D．be perched
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