Final answer:
The question regarding the ultimate CO₂ level for natural gas combustion in relation to excess air temperature is unclear without proper context or a combustion diagram. General information from environmental science indicates the benefits of substituting natural gas for coal due to lower CO₂ emissions and higher energy density.
Step-by-step explanation:
The original question seems to be regarding the combustion of natural gas and the determination of an 'ultimate CO₂ level.' However, it appears that there might be some confusion or miscommunication in the question, as ultimate CO₂ levels typically relate to combustion efficiency and the specific conditions under which combustion takes place, rather than just the excess air temperature. The excess air level is used to ensure complete combustion, which would optimize the efficiency and minimize CO₂ levels.
Considering the provided reference information, there is not enough context to definitively answer the original question with one of the provided choices (A) 9.9%, (B) 12.9%, (C) 8.9%, or (D) 11.9%. Still, based on general concepts in combustion and environmental science, substituting natural gas for coal can reduce CO₂ emissions due to its lower CO2-to-fuel mass ratio and higher energy density, as illustrated by the data provided comparing kcal/g values and CO₂-to-fuel mass ratios. Moreover, effective strategies for reducing fossil fuel use and transitioning to lower-emission energy sources can have significant impacts on reducing CO₂ accumulation in the atmosphere and the associated temperature rise due to the greenhouse effect.