—Look! The man at the gate        be our headmaster. He stands there every morning.

—No, it        be him. He is holding a meeting in the office now.

A．must； can’t

B．must; can

C．can’t; mustn’t

D．can’ t; must

T细胞和B细胞发生免疫应答的场所是（）。

A.骨髓

B.外周免疫器官

C.中枢免疫器官

D.胸腺

E.血液