It has been announced recently that all the schools________ not have the students attend school if the

students have high temperatures.[ ]

A. need  

B. shall  

C. could  

D. ought to

由于空中交通管制工作上的失误，造成飞行取消、返航、备降，属于（）。

A.管制差错

B.管制事故征候

C.管制事故

D.管制严重差错