In the month of September, in Britain, you may see large numbers of birds【C1】______ on roo

fs and telegraph wires. These birds are swallows. They are【C2】______. together because, very soon, they will be flying【C3】______ to much warmer lands, where they will find【C4】______the small flying insects on which they【C5】______. There are no such insects【C6】______in Britain during the winter; it is【C7】______cold for them.

The swallows settle, fly off, swoop, and【C8】______again. This they do many times, for they are making short【C9】______ flights in order to be fit for the long journey【C10】______them.

【C11】______of these migrating birds leave Britain in the autumn. They fly【C12】______for hundreds of miles【C13】______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【C14】______.

In the spring of the following year they【C15】______the long and tiring journey back to Britain. They return to the identical barn or tree in the【C16】______district which they had left the【C17】______autumn. How do these birds find their【C18】______there and back over such vast distances？ Nobody knows exactly【C19】______, but it has something to do【C20】______ winds and air currents.

【C1】

A．being perched

B．perched

C．being perching

D．be perched