# The length of normal to the curve $y = \frac{1}{2} a(e^{x/a} + e^{-x/a})$ is :
( A ) $\frac{y}{a}$
( B ) $\frac{y^2}{a}$
( C ) $\frac{a}{y}$
( D ) $\frac{a}{y^2}$