One can't learn a foreign language well _______ he studies hard.

[      ]

A. though

B. but

C. if

D. unless

适用于制造铆钉、螺栓的钢丝是( )。

A．热镀锌低碳钢丝

B．冷顶锻用碳素结构钢丝

C．冷顶锻用合金钢丝

D．高碳钢丝