单选题We spent many years 

问题补充：

单选题Wespentmanyyearsstudyingtheformationofrocks,experiencingmanysufferingsandevendealth.Butourhardwork_____intheend,andwemadeit.A.paid offB.paid backC.given awayD.came out

答案：

A解析pay off成功;得到回报。“最后我们的努力得到了回报, 我们成功了”