白喉外毒素的A亚单位有1个（）区，B亚单位上有1个（）区和1个（）区.
参考答案：催化、受体结合、转位
The teacher was absorbed in writing something, only _____ now and then to refer to a book or another.
A．to stop
B．stopped
C．stopping
D． having stopped
肝伤害的中毒表现为（）
A.纳差乏力
B.恶心厌油
C.肝功能异常
D.胆红素升高
E.皮肤黄染
