Which response indicates "very retarded timing" in terms of exhaust constituents?

When discussing "very retarded timing," it's essential to understand the relationship between ignition timing and the resulting exhaust constituents. Retarded ignition timing refers to delaying the spark that ignites the air-fuel mixture, which can lead to incomplete combustion and altered levels of various exhaust emissions.

An increase in oxygen (O2) in the exhaust gas is a strong indicator of incomplete combustion. When timing is retarded significantly, the combustion process becomes less efficient, leaving unburned fuel and resulting in more O2 present in the exhaust. Essentially, because the fuel-air mixture does not burn completely, there is excess oxygen in the exhaust that did not get consumed during combustion.

The other responses, while they reflect potential changes in emissions, do not specifically highlight the relationship as clearly as an increase in O2 does. For instance, an increase in hydrocarbons (HC) may occur with retarded timing, but it does not directly indicate the extent of the timing retardation like the presence of unburned oxygen. Similarly, changes in CO and CO2 levels can also occur, but they do not provide as direct and compelling evidence of very retarded timing as the increase in O2 does. Thus, the indication of "very retarded timing" is best represented