巯基乙醇是强还原剂，可将带有二巯键的IgM由19S降到7S的IgM，因而会减少()

A．与Fc受体结合的能力

B．结合补体的能力

C．通过胎盘的能力

D．凝集红细胞的能力

E．结合抗原的能力

—I called you at 4:50 yesterday afternoon, but no one answered.

—Sorry. I_______with my friends at that time.

A．swam

B．swim

C．was swimming

D．will swim