Which of the Following Statements Describes Why Nitrogen Fixation Is

Which of the following statements describes why nitrogen fixation is a difficult process?

A) there is so little nitrogen in the atmosphere
B) nitrogen is very unreactive, largely due to its triple bond
C) nitrogen exists in the atmosphere primarily as its oxides which are very unreactive
D) of the high polarity of nitrogen molecules preventing them from dissolving in biological fluids, such as those inside cells
E) of the extreme toxicity of nitrogen