939.The Niagara Waterfalls,____ is shaped like a semicircle,are 670 meters wide,and fall 56 meters in an awesome white sheet of water.
A.what
B.where
C.the largest of which
D.in which
940.Mr.Smith,____ it so happened,was away on business that day.Maybe I should have phoned him before I came here.
A.what
B.who
C.that
D.as
939.答案是C;the largest of which引导非限制性定语从句，修饰The Niagara Waterfalls。
940.答案是D;此题为as引导的非限制性定语从句，其中as指代整个主句Mr.Smith was away on business that day。
