Which gas is the primary contributor to global warming?

A: carbon dioxide

• Carbon dioxide ($\text{CO}_2$) is the most significant anthropogenic (human-caused) greenhouse gas and the major contributor to global warming. It traps heat in the Earth's atmosphere, leading to an increase in global temperatures.
• Oxygen ($\text{O}_2$) and Nitrogen ($\text{N}_2$) are the two most abundant gases in the Earth's atmosphere, but they are not greenhouse gases and do not contribute to global warming.
• Hydrogen ($\text{H}_2$) is a very light gas and is not a significant greenhouse gas.

The final answer is $\text{A: carbon dioxide}$