Because the parabolic opening is upward, and the function value can be less than 0.
So (-1) 2-4a >: 0
0 & lta & lt 1/4
f(m)=m^2-m+a<; 0
[ 1-√( 1-4a)]/2 & lt; m & lt[ 1+√( 1-4a)]/2
When x=[ 1-√( 1-4a)]/2
y=0,
So x < [1-√( 1-4a)]/2, y >;; 0
Because 0
So 0
So 0
1/2 & lt; [ 1+√( 1-4a)]/2 & lt; 1
So 0
So- 1
So f (m- 1) >: 0.
Choose B.