所有基本的抗体分子由（）构成。
A.4个多糖
B.4个肽链
C.4个酶分子
D.4个ATP分子
参考答案：B
Great changes have taken place in that school. It is no longer ______ it was 20 years ago, _____ it was so poorly equipped.
A．what; when
B．that; which
C．what; which
D．which; that
四级电力供应预警信号对应缺额为（）。
A.20%以上
B.10%～20%
C.5%～10%
D.5%以下
