Protostars with Masses of Less Than About 4 Times the Mass

Protostars with masses of less than about 4 times the mass of the Sun evolve at approximately constant surface temperature through most of their pre-main-sequence lifetimes. This is because:

A) they contract too rapidly for their temperature and luminosity to change significantly.
B) they are not massive enough to contract significantly and luminosity remains constant.
C) the luminosity increases as the surface area decreases.
D) the luminosity decreases as the surface area decreases.