We spent many years studying the formation of rocks, experiencing many sufferings and even dealth .But our hard work _____in the end, and we made it.

A．paid off

B．paid back

C．given away

D．came out

施工升降机钢丝绳的安全系数不得小于12，直径不得小于（）mm。

A、6

B、9

C、12

D、15