下列哪项不是老年人高血压的临床特点（）

A.年龄超过60岁

B.半数以上为单纯性收缩期高血压

C.一部分属于收缩压和舒张压均增高的混合型高血压

D.发生靶器官损害较常见

E.血压波动小，不易发生体位性低血压

It is obvious to the students ____ they should get well  prepared for their future.    [ ]

A. as              

B. which      

C. whether  

D. that