If the parabola y \equal{} \minus{}x^2 \plus{} bx \minus{} 8 has its vertex on the x-axis, then b must be:
<spanclass=′latex−bold′>(A)</span>a positive integer<spanclass=′latex−bold′>(B)</span>a positive or a negative rational number<spanclass=′latex−bold′>(C)</span>a positive rational number<spanclass=′latex−bold′>(D)</span>a positive or a negative irrational number<spanclass=′latex−bold′>(E)</span>a negative irrational number