Why Did the Solar Nebula Flatten into a Disk

Why did the solar nebula flatten into a disk?

A) The interstellar cloud from which the solar nebula formed was originally somewhat flat.
B) It flattened as a natural consequence of collisions between particles in the nebula, changing random motions into more orderly ones.
C) The force of gravity pulled the material downward into a flat disk.
D) As the nebula cooled, the gas and dust settled onto a disk.