Answer: C is the best answer
this question is weirdly phrased, I picked C through process of elimination. Here's my reasoning:
It cannot be A because although the paragraph explains what greenhouses are, that it not the point of the passage. It is not B because even though the greenhouse effect is mentioned it does not explicitly say it is changing the atmosphere - just the temperature of the Earth. It is not D because according to the paragraph the greenhouse effect is exactly why the suns heat cannot escape.