Why Are Gasses Exchanged in the Respiratory Systems of Insects

Why are gasses exchanged in the respiratory systems of insects and fish?

A) Insects and fish require oxygen as a reactant for cellular respiration.
B) Insects use nitrogen gasses as both a reactant and product of cellular respiration,and fish use oxygen as a reactant and carbon dioxide as a product of cellular respiration.
C) Insects and fish create carbon dioxide as a waste product of cellular respiration.
D) Both A and C.
E) All of the above are hurdles.