What is in mol/L of a solution that is 0.20 M in $CH_3COONa$ and 0.10 M in $CH_3COOH$ Ka for $CH_3COOH=1.8\times10^-5$

Question

( A ) $3.5\times10^{-4}$
( B ) $1.8\times10^{-5}$
( C ) $1.1\times10^{-5}$
( D ) $9.0\times10^{-6}$