If $C_n$ is the coefficient of $x^n$ in the expansion of $(1+x)^n$, then $C_1 + 2C_2 + 3C_3 + ...+nC_n$ is equal to :

Question

( A ) $n. 2^{n+1}$
( B ) $n. 2^{n-1}$
( C ) $n. 2^n$
( D ) $2^n$