AHSME 1951 Problem 18

The expression $21x^2 +ax +21$ is to be factored into two linear prime binomial factors with integer coefficients. This can be done if $a$ is: $\textbf{(A)}\ \text{any odd number} \textbf{(B)}\ \text{some odd number} \textbf{(C)}\ \text{any even number}$ $\textbf{(D)}\ \text{some even number} \textbf{(E)}\ \text{zero}$