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

Percoll分层液法常用来分离（）。

A.单核细胞和淋巴细胞

B.白细胞

C.T细胞和B细胞

D.红细胞

E.单个核细胞