---Have you seen _____ Audi car? I parked it here two hours ago, but now it’s gone!

---Is it _____ black one? A young man has just driven a black car away.

A．an…a

B．the…the

C．the… a

D．an…the

是B细胞上的主要表面膜Ig，作为抗原受体而引发免疫应答的是（）

A.IgE

B.IgG

C.IgD

D.IgM

E.IgA