The old man, _______ abroad for twenty years, is on the way back to his motherland.

A．to work　

B．working 

C．to have worked

D．having worked

关于药敏试验的叙述，不正确的是（）

A.制备琼脂平板时，加入抗菌药物的温度是50℃左右

B.MBC指最小杀菌浓度

C.MH琼脂平板琼脂厚度一般为3mm

D.抑菌环直径越大，MIC越大

E.MIC是指最小抑菌浓度