There was a __________look on her face because a________ problem _________the little girl.

A．puzzled; puzzled; puzzled

B．puzzling; puzzling; puzzled

C．puzzled; puzzling; puzzled

D．puzzling; puzzled; puzzling

实验中由针对抗原的第一抗体和用荧光标记针对第一抗体的第二抗体相继作用的是（）

A.直接法

B.间接法

C.夹心法

D.补体结合法

E.双标记法