-Listen! Helen is singing in the next room.

-It __________ be Helen. She has gone to Beijing.[ ]

A. can't

B. mustn't

C. may

D. should

A．Opium being made legal in mid-nineteenth-century China.

B．The end of prohibition of alcohol in America in the 1920s and 1930s.

C．Drug pushers making billions of dollars each year.

D．More money being needed in education and medical care.