— Tom graduated from college at a very young age. 

— Oh, he _____ have been a very smart boy then. [ ]

A. could

B. should

C. might

D. must

预防小儿脊髓灰质炎，下列哪项正确（）

A.免疫原为灭活的脊髓灰质炎病毒疫苗

B.接种方式为口服

C.初种年龄为生后2个月

D.冷开水送服

E.接种后可有低热或轻泻