The expression 21x^2 \plus{} ax \plus{} 21 is to be factored into two linear prime binomial factors with integer coefficients. This can be one if a is:
<spanclass=′latex−bold′>(A)</span>any odd number<spanclass=′latex−bold′>(B)</span>some odd number<spanclass=′latex−bold′>(C)</span>any even number<spanclass=′latex−bold′>(D)</span>some even number<spanclass=′latex−bold′>(E)</span>zero