Thermodynamics

Given (i)  C (graphite) + O₂(g) $$\to$$ CO₂(g); $$\Delta$$rH$$^\Theta$$ = x kJ mol^$$-$$1 (ii)  C(graphite) + $${1 \over 2}$$O₂(g) $$\to$$ CO(g); $$\Delta$$rH$$^\Theta$$ = y kJ mol^$$-$$1 (iii)  CO(g) + $${1 \over 2}$$ O₂(g) $$\to$$ CO₂(g); $$\Delta$$rH$$^\Theta$$ = z kJ mol^$$-$$1 Based on the above thermochemical equations, find out which one of the following algebraic relationships is correct?