In the month of September, in Britain, you may see large numbers of birds （1)_____ on roof

In the month of September, in Britain, you may see large numbers 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)_____ the small flying insects on which they (5)_____. There are no such insects (6)_____ in Britain during the winter; 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 journey (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．perched

C．being perching

D．be perched