Correct Option: C

High Carbon Monoxide (CO) content in exhaust gas of automobiles usually indicate a fuel mixture richer than ideal (rich mixture - air fuel ratio below 14.7).High CO levels result from inadequate O2 supply needed for complete combustion that is caused by a too rich mixture - too much fuel or not enough air. This is most seen during : l Idle running or low idle speed; l Improper float settings in carbureted vehicles; l Dirty or restricted air filters; l Excessively dirty or contaminated oil l Improper operation of the fuel delivery system; etc.