Ans: B
Since $a-6$, $a$ and $a+5$ form a geometric sequence, then we have
Since $a-6$, $a$ and $a+5$ form a geometric sequence, then we have
$\begin{array}{rcl}
\dfrac{a}{a-6} & = & \dfrac{a+5}{a} \\
a^2 & = & (a+5)(a-6) \\
a^2 & = & a^2 -a -30 \\
a & = & -30
\end{array}$
Therefore, the common ratio
$\begin{array}{cl}
= & \dfrac{a}{a-6} \\
= & \dfrac{-30}{-30-6} \\
= & \dfrac{5}{6}
\end{array}$
