若方程组 中x与y的值相等,则k等于( )
A:1或-1
B:1
C:5
D:-5
函数f(x)=2x-x2,(0≤x≤3)x2+6x,(-2≤x<0)的值域是 ()
The factory ______ its waste into the river, but now it deals with the waste in a new way.
控制生物性状的遗传物质的结构单位和功能单位是( )