I want to buy ____________ socks.[ ]
A. a pair of
B. two
C. a piece of
答案：A
如图甲所示，在一个向右行驶的车厢内有一高h的货架，货架边缘有一小球，当车突然加速行驶时，小球从货架边缘脱落，若小球下落过程中未与车厢的后壁相碰，则以地面为参考系，小球下落时的运动轨迹是图乙中的
---We can't be successful _____ we keep on working hard.
---I agree with you.
A．if
B．because
C．unless
D．until
