The greenhouse effect is primarily caused by which human activity?

• A. Deforestation has no effect on greenhouse gases
• B. Ozone formation cools the lower atmosphere
• C. Burning fossil fuels increases CO2 and traps heat
• D. Increased solar radiation directly warms Earth's surface

Answer: (C)

Greenhouse gases trap heat in the atmosphere by absorbing infrared radiation, which keeps Earth warmer than it would be otherwise. Burning fossil fuels releases large amounts of CO2, increasing the concentration of these gases in the air and capturing more heat. That strengthens the greenhouse effect and leads to warming. Other options miss the mechanism or the human contribution: deforestation does affect greenhouse gases by reducing the land’s ability to take up CO2, so it’s not "no effect"; ozone formation doesn’t simply cool the lower atmosphere; and more solar radiation is a natural energy input, not a human-caused driver of the greenhouse effect.