____  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

下列氨基酸中能转化生成儿茶酚胺的是

A．天冬氨酸

B．色氨酸

C．酪氨酸

D．脯氨酸

E．蛋氨酸