Hydrogen "Burning" by Fusion Reactions Occurs Only in the Deep

Hydrogen "burning" by fusion reactions occurs only in the deep interior of the Sun (and other stars) because

A) the density there is sufficiently low that hydrogen atoms can collide with each other often enough.
B) this is the only place in the Sun where there is sufficient hydrogen.
C) only in the core is the temperature low enough and the density high enough to allow fusion to occur.
D) the requisite conditions of high temperature and high density occur only there.