Question

The use of carbon 12 scale has superseded the oldest scale of atomic mass based on

Answer 1

Originally Answered:
Its based on C-12 because, I guess, that gave results between the 2 definitions used until 1961 based on oxygen. See

It had to be based on a single isotope (unlike pre-1961 definition used by chemists) because the average atomic weight of pure elements can vary because samples from different source may have a different mix of isotopes. (They could have used gold or another element with only one stable isotope but that would have changed the numbers by too much.)