Question

【题目】Children should be warned that it's dangerous to open the microwave oven ________ it is still working.
A.until
B.after
C.before
D.while

Answer 1

【答案】D
【解析】A. until直到；B. after在…之后；C. before在…之前；D. while在…期间，而，虽然，与…同时。句意：应该警告孩子，当微波炉工作的时候，打开它是很危险的。根据句意可知选D。