教材中的下列实验设立了对照组的是( )
—__________ does your father work?
—He works in a factory.
设椭圆的方程为x2a2﹣y2b2=1(a>b>0),右焦点为F(c,0)(c>0),方程ax2+bx﹣c=0的两实根分别为x1,x2,则P(x1,x2)( )