Let ' $a$ ' be a positive constant number. Consider two curves $C_{1}: y=e^{x}, C_{2}: y=e^{a-x} .$ Let $S$ be the area of the part surrounding by $C_{1}, C_{2}$ and the $Y$ -axis, then $\lim _{a \rightarrow 0} \frac{S}{a^{2}}$
equals
(a) 4
(b) $1 / 2$
(c) 0
(d) $1 / 4$