—Do you know about the accident which happened in the center of the city?

—Yes. But it was some time           we realized the truth.

A．before

B．when

C．since

D．until

提高滚珠导轨承载能力的最佳方法是_______。

A.增大滚珠直径

B.增加滚珠数目

C.增加导轨长度