The coefficient of the middle term in the expansion of $(1+x)^{40}$ is equal to :

Question

( A ) $\frac{1.3.5....39}{(20!)}$
( B ) $\frac{40 !}{(20!)^2}$
( C ) $\frac{1.3.5....39}{(20!)} . 2^{20}$
( D ) $4! 2^{20}$