Which of the following best explains why the lower portion of the troposphere is warmer than the upper portion?
**A**
The lower portion of the troposphere is separated by the ozone layer from the upper portion, so the lower portion absorbs more sunlight, increasing the temperature.
**B**
The air in the lower portion of the troposphere is less dense than the air in the upper portion, leading to a higher temperature in the lower portion.
**C**
The surface of Earth absorbs sunlight, heating the lower portion of the troposphere more than the upper part.
**D**
The troposphere is the highest layer of the atmosphere, so the lower portion is warmed more than the upper part by rising air.