The scientist _____ Canada and he will give us a talk when he _____ back.

A．has gone to/comes

B．has been to/will come

C．has gone to/will come

D．has been to/comes

混合感染时选用（）

A.窄谱抗生素

B.广谱抗生素

C.抑菌性抗生素

D.杀菌性抗生素

E.联合应用抗生素