It is obvious to the students they should get well prepared for their future.
A. as
B. which
C. whether
D. that
网友回答
A. as
B. which
C. whether
(答案→)D. that
解析：考查名词性从句用法。本题中的it是形式主语，真正的主语是后面的that they should get well prepared for their future. that在句中不充当任何成分，只起引导作为。故D正确。