The children had to repeat that story twice,________they?
A:didn't
B:hadn't
C:did
D:had
已知函数f(x)=cosxsinx,给出下列四个说法:①若fx1=-fx2,则x1=-x2;②fx的最小正周期是2π;③fx在区间-π4,π4上是增函数;④fx的图象关于直线x=3π4对称.其中正确说法的个数为( )
水在重要的自然资源,也是生命之源.