Which of the following best describes why the solar nebula flattened into a disk?

a. as a natural consequence of collisions between particles in the nebula, changing random motions into more orderly ones.
b. because the nebula cooled and the gas and dust settled onto a disk.
c. because the interstellar cloud it came from was already almost flat.
d. because gravity pulled the material downward into a flat disk.
e. because Jupiter gravitationally flattened the disk.