Natural Science CLEP Prep Practice Exam

The most sun radiation in the Earth's atmosphere occurs during the day because that is when the sun is visible and most actively emitting radiation. During the night, the sun is not visible and therefore not emitting as much radiation. Before the sun rises and after the sun sets, there is less direct exposure to the sun's rays, so the amount of radiation in the atmosphere is also lower compared to daytime. Therefore, A is the most accurate answer.