$\begin{align*} \displaystyle \lim_{n \to \infty} \displaystyle\sum_{r=1}^{n} \frac{1}{n} e^{^{\frac{r}{n}}} \end{align*}$ is

Question

( A ) $e-1$
( B ) $e+1$
( C ) $1-e$
( D ) $e$