________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

下面有关天丝的说法正确的是（）

A.天丝是一种天然纤维

B.天丝是将竹子做成竹浆粕然后纺丝制成的

C.天丝是一种人造纤维

D.在生产天丝过程中会对环境造成一定的污染