In the binomial expansion of ${\left( {a - b} \right)^n},\,\,\,n \ge 5,$ the sum of ${5^{th}}$ and ${6^{th}}$ terms is zero, then $a/b$ equals

Question

${{n - 5} \over 6}$

${{n - 4} \over 5}$

${5 \over {n - 4}}$

${6 \over {n - 5}}$

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