Why Can't Plants Use N2 Directly as a Nutrient

Why can't plants use N2 directly as a nutrient?

A) Plants lack the enzymes necessary to convert N2 into NO3- or NH4+.
B) Plant hormones are not capable of carrying out nitrogen fixation.
C) Most soils are depleted of N2 as a nutrient.
D) N2 diffuses out of leaves as quickly as it diffuses in, so it has to be "fixed" to remain in leaves.
E) The N2 molecule is too large to enter the root hairs.