AHSME 1961 Problem 7

When simplified, the third term in the expansion of $(\frac{a}{\sqrt{x}}-\frac{\sqrt{x}}{a^2})^6$ is: $\textbf{(A)}\ \frac{15}{x} \textbf{(B)}\ -\frac{15}{x} \textbf{(C)}\ -\frac{6x^2}{a^9} \textbf{(D)}\ \frac{20}{a^3} \textbf{(E)}\ -\frac{20}{a^3}$