Since I knew how hard the life they lead is I have understood they need so much money.A.

问题补充：

Since I knew how hard the life they lead is, I have understood they need so much money.

A. how

B. why

C. whether

D. when

答案：

A. how

(答案→)B. why

C. whether

D. when

解析：考查连接副词：A. how怎样B. why为什么，C. whether 是否，D. when什么时候，句意：自从我知道他们的生活有多难，我一直理解他们为什么要这么多钱。选B。