It is obvious to the studentsthey should get well prepared for their future.A as b whichC

问题补充：

It is obvious to the studentsthey should get well prepared for their future.

A,as b,whichC,whether D,that

答案：

解析：通过分析句子结构，可知句子的真正主语是后面的主语从句，it是形式主语，且引导词在主语从句中没有作任何成分，且没有任何实际意义，故用that。