The air in Beijing is getting much ______ now than a few years ago.[ ]

A. clean              

B. cleaner

C. cleanest        

D. the clcanest

有以下程序：
main()
int i=0, s=0;
do
if(i % 2)
i++; continue;
i++;
s+=i;
while(i＜7);
printf("% d\n", s);

执行后的输出结果是______。

A．16

B．12

C．28

D．21