AHSME 1961 Problem 32

A regular polygon of $n$ sides is inscribed in a circle of radius $R$. The area of the polygon is $3R^2$. Then $n$ equals: $\textbf{(A)}\ 8 \textbf{(B)}\ 10 \textbf{(C)}\ 12 \textbf{(D)}\ 15 \textbf{(E)}\ 18$