单项选择题
A．0.045% B．0.035% C．0.030% D．0.020%
A.普通钢 B.优质钢 C.高级优质钢 D.特级优质钢
A.0.10～0.20% B.0.20～0.30% C.0.30～0.70% D.0.70～1.00%