A chemistry question about volume?

You are told that both reactant volumes are measured at the same temp and pressure so assuming ideality 2vols of CO will react with 1vol of O2 to give 2vol of CO2. There is not enough O2 in the reaction to convert all the CO (5vol of CO would need 2.5vol of O2) 2vol of O2 will convert 4vol of CO to give 4vol of CO2 leaving 1vol of CO unconverted. So your answer is B.4