In the reaction
`PCI_(5)(g)hArr PCI_(3)(g)+CI_(2)(g)`
the equilibrium concentrations of `PCI_(3)` and `PCI_(3)` are `0.4` and `0.2 mol^(-1)`, respectively. If the value of `K_(c)` is `0.5`, what id the concentration of `CI_(2)` in moles per litre ?
A. `2.0`
B. `1.5`
C. `1.0`
D. `0.5`