Mr. Bob, ______ as a manager for many years, found it hard to be an ordinary clerk again.

A．having worked

B．worked

C．to have worked

D．working

苯丙酮酸尿症患者，尿中排出大量苯丙酮酸、苯丙氨酸，因为体内缺乏哪种酶（）

A.磷酸吡哆醛

B.酪氨酸转氨酶

C.苯丙氨酸羟化酶

D.多巴脱羧酶

E.酪氨酸羟化酶