In an Open Cluster, the Sizes of Some of the Lower-Mass

In an open cluster, the sizes of some of the lower-mass stars may have been limited when the surrounding material the stars were accreting was blown away by ultraviolet radiation. What is the source of this UV radiation?

A) The radiation sweeps generally through the Galaxy.
B) The radiation comes from supernova explosions outside the cluster.
C) The more massive O and B stars in the cluster evolve more quickly and produce ultraviolet radiation, while the less massive stars are still evolving toward the main sequence.
D) The ultraviolet radiation is emitted by the low-mass stars themselves when they begin hydrogen burning.