TOEFL Reading: ETS-TOEFL阅读机经 - U5Y82JK5I6T_58C_L$

According to paragraph 3, which of the following best explains why the inner part of the nebula became hotter as the nebula contracted? A. The nebula rose in temperature as its speed of rotation increased. B. Atomic particles in the nebula gave off more heat when they became energized. C. The dust grains broke up into smaller parts, allowing more light from the Sun to reach the inner portion of the nebula. D. As the nebula collapsed, its gravitational energy was changed into heat energy