我国采用()信令网结构。
A.五级
B.四级
C.三级
D.二级
参考答案:C
——Why doesn't he buy you a drink?
——I don't know why, but it isn't _______ he had no money.
A.even if
B.as if
C.that
D.because
简述中国新石器时代的石器文化的区域分布及特征。