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.