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->0) s/a^2` equals (A) 4 (B) `1/2` (C) 0 (D) `1/4`