问题
选择题
As he was about to go out to search for Mary, she happened ____.
A．to coming in
B．to come in
C．came in
D．to have come in
答案
答案：B
考查happen用法。Happen发生，碰巧；happen to do sth碰巧做某事；句意：当他正要出去找Mary的时候，她碰巧进来了。D项表示已经完成的事情，不符合句意。故Ｂ正确。