Order the Following Metabolic Innovations to Reflect the Most Likely

Short Answer

Order the following metabolic innovations to reflect the most likely order in which they evolved during biological evolution. Your answer would be a four-letter string composed of letters A to D only, e.g. BDCA.
(A) Using light energy to generate reducing power
(B) Splitting of water molecules (an electron source)
(C) Fermentation of organic material
(D) Reduction of molecular oxygen (a terminal electron acceptor)