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

18. Once _____ what to do, these computers will show great power in carrying it out.

A．to teach　　　　　 B．teaches　　　　 C．teaching　　　 D．taught