A is a sufficient assumption, not a neccessary one.
it could be that a rattlesnake molts 30 times a year, and you would still be able to use rings to accurately tell its age.
the key is that the molting must be consistent. if you did not assume E, then it would be impossible to use rings as an accurate dating tool because you would never know if they were developing at a constant rate.
Where do these terms "sufficient" and "necessary" come from (with respect to assumptions)? I see them with conditional statements but have never heard of them used with assumptions. Is there some obvious connection I'm missing?