— The book isn't easy for Jack to understand,   is it?

— ________.  His foreign language is far better than expected.  [ ]

A. No,   it isn't  

B.  I'm afraid not

C. I don't think so  

D.  Yes,   it is

环丝氨酸成人剂量1g/d时，建议同时服用维生素B6，每服用250mg的环丝氨酸可给予多少量的维生素B6。（预防神经系统毒性）（）

A、20mg

B、30mg

C、40mg

D、50mg