几个高二英语完成句子~

几个高二英语完成句子~
we should consider what use ___(能利用)such a book
All flights _____(被取消)because of the snowstorm,many passengers could do nothing but take the train (cancel) 答案是“having been canceled”为什么要用完成时啊
______(排队等了半个小时),the old man suddenly realized hr had left the cheque in the car.(wait)答案是“Waiting in the queue for half an hour”为什么不用“Having been waiting?”