The Hydrogen in the Outer Layers of the Sun Cannot

The hydrogen in the outer layers of the Sun cannot undergo fusion because the

A) hydrogen in the outer layers is fundamentally different from the hydrogen in the core.
B) hydrogen in the outer layers is pure,and hydrogen must be mixed with helium,as it is in the core,in order to undergo fusion.
C) temperature in the outer layers is too low to allow the hydrogen nuclei to come close enough together to undergo fusion.
D) hydrogen atoms in the surface layers are not ionized,as they are in the core,and ionization is one requirement for nuclear fusion.