It is these poisonous products _____ can cause the symptoms of the flu, such as headache and aching

muscles. [ ]

A. who

B. that

C. how

D. what

计算，其中Ω为z²=x²+y²，z=1围城的立体，则正确的解法是（）。

A.A 

B.B 

C.C 

D.D