Which two atoms are isotopes of each other?

Which substance can be broken down by chemical means

Which best describes the present-day universe? opaque, expanding very slowly, stars produce heavy elements transparent, expanding at an accelerated rate, stars produce heavy elements opaque, expanding at an accelerated rate, stars produce only hydrogen and helium transparent, expanding very slowly, stars produce only hydrogen and helium

If you had to research a particular disease or area of concern in veterinary medicine and science, which one would you choose? why?