Which of the following describes a major role of gravity in star formation? a. gravity causes an interstellar cloud fragment to expand rapidly until it is large enough to form a star at its center. b. gravity compresses gases at the center of a solar nebula until temperatures are high enough for nuclear fusion to occur. c. the gravitational forces of planets push gases from their atmospheres to form a star at the solar system's center. d. asteroids passing by a solar system are pulled in by planets' gravities to form a star at the center of the solar system.