Oil and water can separate by utilizing a separating funnel, oil and water are totally insoluble in one another. The water structures lower while the oil forms the upper layer.
The most abundant of all of the isotopes of an element will be the one who's mass the mass of element is closest to. In this case, the mass of atomic carbon is closest to the mass of carbon-12. Thus, Carbon-12 is the most abundant isotope.
I'm pretty sure this is a question about your opinion so there is no wrong answer! Just think about the question and if you were in those shoes. There is no right or wrong answer! :)
The other option that would make the most sense would be electrons, however the mass of an electron is so small that is basically negligible, so it's not included in the atomic mass.
