牛顿第二定律的数学表示式是
A. F=mt
B. F=mw
C. F=ma
D. F=dp/dx
He often helps his mother do the housework ________
A. ,
B. :
C. ;
D. .
It was raining hard ________ they could not work in the fields.
A. ;
B. :
C. ,
D. !
I will go to Shanghai ________ Beijing and Shenzhen.
A. ;
B. ,
C. :
D. ?