自然界中的菱铁矿实际含铁量在30%～40%之间，含有害杂质（）。
A.一般
B.较多
C.较少
参考答案：C
That boy is not to go to school． He is only three years old．
A．young enough
B．old enough
C．enough young
D．enough old
低碳钢焊丝中含碳量规定小于（）。
A.0.25%
B.0.2%
C.0.15%
D.0.1%
