$A$ saturated solution of $BaSO_4$ at $25^\circ C$ is $4 \times 10^{-5} \ M$. The solubility of $BaSO_4$ in $0.1 \ M \ Na_2SO_4$ at this temperature will be:

• A
  $1.6 \times 10^{-9} \ M$
• B
  $1.6 \times 10^{-8} \ M$
• C
  $4 \times 10^{-6} \ M$
• D
  $4 \times 10^{-4} \ M$