问题补充:
in the qucue for half an hour ,the old man auddenly realized he had left the cheque in the car.
A. Waiting
B. To wait
C. Having waited
D. To have waited
答案:
A. Waiting
B. To wait
(答案→)C. Having waited
D. To have waited
解析:有明显的先后关系,故要用V-ing的完成式。