China has _____ population of more than 13 billion, and about 60 percent of them _____ peasants.   [ ]

A. the; are    

B. the; is      

C. a; are      

D. a; is

如图所示，在瓶的a、b两处各扎一个小孔，并将孔封住．在瓶中注满水，打开 a、b两孔后观察到图示的情况，这说明：①　      ；②　      ．