— Did you find the missing couple in the mountain yesterday?

— No, but we ____ to get in touch with them ever since. [ ]

A. have tried

B. have been trying

C. had tried

D. had been trying

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．丙氨酸

E．蛋氨酸