--You were brave enough to raise objections at the meeting.

---Well, now I regret____ that.

A．to do

B．to be doing

C．to have done

D．having done

沉淀反应形成的反应现象（）（）

A.肉眼可见

B.肉眼不可见

C.必须用精密仪器测定

D.必须染色才可见

E.经电泳后可见