If \( PO_4^{3-} \) ion is added in an aqueous solution of \( AgNO_3 \) (concentration 0.1M), then at what concentration of \( PO_4^{3-} \) \( Ag_3PO_4 \) starts to form precipitates? \( K_{sp}(Ag_3PO_4) = 1.3 \times 10^{-20} \) .

A \( 2.6 \times 10^{-17} \)

B \( 3.9 \times 10^{-17} \)

C \( 1.3 \times 10^{-17} \)