Out of the following given choices;
a) digestive and circulatory
b) excretory and digestive
c) reproductive and circulatory
d) immune and muscular.
The answer is A. The main precursor of fatty acids synthesis is acetyl-CoA. It can be formed from the respiration of carbohydrates in the glycolytic pathway. The other precursor is NADPH and NADH which are reducing factors in the body. They require H+ ions. H+ is regulated by the circulatory/respiratory system as pH of blood is regulated.