When Steven suddenly remembered that he had left his mobile in the office, the train was already ________.

A．on the go

B．on the rise

C．on the run

D．on the move

精脒、精胺的前体是（）

A．鸟氨酸

B．谷氨酸

C．组氨酸

D．色氨酸

E．苏氨酸