The Equilibrium Constant (K_p)for the Interconversion of PCl₅ and PCl₃

The equilibrium constant (K_p) for the interconversion of PCl₅ and PCl₃ is 0.0121: PCl₅ (g) PCl₃ (g) + Cl₂ (g)
A vessel is charged with PCl₅ giving an initial pressure of 0.123 atm. At equilibrium, the partial pressure of PCl₃ is ________ atm.

A) 0.0782
B) 0.0455
C) 0.0908
D) 0.0330
E) 0.123