If $(1+x)^n = C_0 + C_1 x+ ... C_n x^n$, then the value of $\displaystyle\sum_{r=0}^{n} \; \displaystyle\sum_{s=0}^{n} \; C_r C_s$ is equal to :

Question

( A ) $2^{n+1}$
( B ) $2^{3n}$
( C ) $2^{2n}$
( D ) $2^n$