11.That winter we ______to write a history of our school.

A.set out　　　 　　　　　　　　　　　　　　　　　　　 B.set about

C.set off　　 　 　　　　　　　　　　　　　　　　　　　 D.set up

20. The tallest building _____ I like most is the one_____windows face the lake.

A. /; which B. /; that　　 C. that; whose　　 　 D. which; whose

19.The roof fell _____ he had time to rush out.

A．until　 　　　　 B．before　　　 　　 C．when　　　　 D．since