—_____ David be the new chairperson of the Students' Union?

—He _____be, as you know, he is quite excellent but I'm not sure.[ ]

A. Can; can't

B. May; must

C. Can; may

D. May; mustn't

方程|x|=2πcosx在（-∞，+∞）内[ ]

A.有且仅有2个根

B.有且仅有4个根

C.有且仅有6个根

D.有无穷多个根