_____ of the students in our class _____ money to the disabled people these days. [ ]

A. Two third; have raised

B. Two thirds; has raised

C. Two three; have raised

D. Two thirds; have raised

观察法的特点是（）

A.高有效性

B.高可靠性

C.时间弹性

D.低费用

E.以上都是