Why Does the Upper Atmosphere Contain Only Very Little Dissociated

Why does the upper atmosphere contain only very little dissociated nitrogen?

A) most of the nitrogen is in the troposphere and not in the upper atmosphere
B) the bond energy of nitrogen is very high and it does not absorb radiation very efficiently
C) nitrogen atoms are extremely reactive and so react with other substances immediately upon their formation
D) the dissociated nitrogen very rapidly diffuses out of the atmosphere and into space
E) There is no N₂ in the upper atmosphere.