Which of the Following Trigger Mechanisms for the Initiation of Stellar

Which of the following trigger mechanisms for the initiation of stellar formation in gas and dust clouds produces many low-mass stars and only a few high-mass, hot O and B stars?

A) passage of the cloud through a black hole
B) compression caused by the passage of the cloud through a galactic spiral arm
C) shock wave from a supernova explosion
D) gravitational condensation of the cloud