小儿急性上呼吸道感染最常见的病原体是（）

A.病毒

B.细菌

C.支原体

D.衣原体

E.真菌

— I don't know          makes her different from others.    

— Confidence, I think.[ ]

A. how is it that  

B. how it is that    

C. what is it that    

D. what it is that