If the coefficient of $x^7$ in $\begin{bmatrix} ax^2 + \begin{pmatrix} \frac{1}{bx} \end{pmatrix} \end{bmatrix}^{11}$ equals the coefficient of $x^{-7}$ in $\begin{bmatrix} ax^2 - \begin{pmatrix} \frac{1}{bx} \end{pmatrix} \end{bmatrix}^{11}$, then $a$ and $b$ satisfy the relation :

Question

( A ) $ab = 1$
( B ) $a + b = 1$
( C ) $a - b = 1$
( D ) $\frac{a}{b} = 1$