(x+1)4的展开式中x2的系数为
A:4
B:6
C:10
D:20
新疆发展农业生产的优势是( )
When H7N9 bird flu appeared, people didn't know __________ doctors could cure it.
动圆C经过点F(1,0),并且与直线x=﹣1相切,若动圆C与直线y=x+22+1总有公共点,则圆C的面积( )