In Stage 6 or 7 of the Formation of a Large

In stage 6 or 7 of the formation of a large cluster of stars, a nebula is formed around thecluster. This happens because:

A) the number of stars is so great and so intense that the gas from the original cloud is ionized.
B) the stars are out of their cocoons of dust and their radiation ionizes the gas from the originalcloud.
C) there are brown dwarfs everywhere in between the stars, so the gas is lit up by their low-intensity light.
D) there are thousands of Sun-like stars with planets around them and the formation of planetsionizes the leftover gas.
E) there are massive O and B stars emitting high energy photons that ionize the remainder of thecloud.