4.0 moles of PCl₅ dissociate at 760 K in a 2 litre flask. PCl₃(g)⇌PCl₃(g)+Cl₂(g) at equilibrium. 0.8 mole of Cl₂ was present in the flask. The equilibrium constant would be

A. 1.0×10⁻¹
B. 1.0×10⁻²
C. 1.0×10⁻³
D. 1.0×10⁻⁻⁴