煤炭开采、自行车、钟表等行业正处于行业生命周期的( )。
A．幼稚期 B．成长期C．成熟期 D．衰退期
参考答案：D
钒触媒床层表面铺合金网或瓷球的主要作用是（）和（）。
_____ in the queue for half an hour, the old man suddenly realized he had left the cheque in the car.[ ]
A. Waiting
B. To wait
C. Having waited
D. To have waited
