Both A and R are correct and R does not explain A
Ecological pyramids were developed by Charles Elton and are, therefore, also called Eltonian pyramids. An ecological pyramid is the representation of energy flow in an ecosystem. It can be upright, inverted or spindle shaped depending on the type of pyramid. There are three types- energy, biomass and number at each trophic level, that is parasitic, aquatic or terrestrial ecosystems.