根据硫、磷含量不同，碳素钢可分为（）
A、普通钢
B、优质钢
C、高级优质钢
D、玛钢
参考答案：A, B, C
生物学的科学探究过程大致包括六个环节：______，______，______，______，______，______．
The brown trousers _______ 70 dollars. And that white pair of trousers _______ 80 dollars.
A．is; is
B．is; are
C．are; are
D．are; is
