In the binomial expansion of $(a-b)^n, n \geq 5$, the sum of $5^{th}$ and $6^{th}$ terms is zero, then $\frac{a}{b}$ equals :

Question

( A ) $\frac{n-5}{6}$
( B ) $\frac{n-4}{5}$
( C ) $\frac{6}{n-5}$
( D ) $\frac{5}{n-4}$