AMC 12 2009a Problem 15

For what value of $n$ is $i + 2i^2 + 3i^3 + \cdots + ni^n = 48 + 49i$?

Note: here $i = \sqrt { - 1}$.

\textbf{(A)}\ 24 \textbf{(B)}\ 48 \textbf{(C)}\ 49 \textbf{(D)}\ 97 \textbf{(E)}\ 98