# If the coefficient of $r^{th}$ term and $(r+1)^{th}$ term in the expansion of $(1+x)^{3n}$ are in the ratio $1 :2$, then $r$ is equal to :
( A ) $\frac{1}{3}(3n+1)$
( B ) $\frac{1}{3} ( 3n+2)$
( C ) $\frac{6}{5} (n+1)$
( D ) $\frac{1}{4} (n+2)$