There Are Several Mechanisms That Can Trigger Star Formation in a Cold

There are several mechanisms that can trigger star formation in a cold, dark nebula. In each mechanism, the key to star formation is

A) bathing the cold, dark nebula in ultraviolet radiation and sweeping away some of the colder material.
B) compressing the gas and dust so that gravitation will overcome the gas pressure.
C) heating the gas so that gas pressure will overcome gravitation.
D) subjecting the dark nebula to an intense magnetic field so that supersonic jets will form.