A. Oxygen-18 is the correct reference for this isotope. As this notation requires the name of the element followed by a dash and the mass number of that corresponding element. The mass number is obtained by adding the number of protons and neutrons together as in this example.
Oxygen
Photosynthesis
ATP (chemical energy)
It all depends what theory it is most are supported by really good evidence but they just don't have all the evidence so it can't be proven a fact at that time
