Astronomers Studying Regions Like the Orion Giant Molecular Cloud Have

Astronomers studying regions like the Orion Giant Molecular Cloud have observed that a wave of star formation can move through them over many millions of years. What sustains such a wave of star formation in a giant molecular cloud?

A) radio waves from complex molecules move slowly through the cloud, causing stars to form
B) when a group of stars form, they remove so much material from the cloud that only a big empty place is left, into which new matter from other clouds falls, making more stars
C) when massive stars form, their ultraviolet radiation and later their final explosions compress the gas in the cloud and cause a new group of stars to form
D) when giant molecular clouds collide with each other, they do so not just once, but many times
E) the dust in these clouds is so heavy, it is always settling inward toward the cloud's center causing star formation in its wake