Help plsss!
If the average atomic mass is 39.948amu, which isotope is more abundant?

It depends on which element you are talking about. I appears that no element has an average atomic mass of that number. So, for the question, if it assumes that there are two isomers with 39 amu and 40 amu, then the 40 amu one would be more abundant. But again, there are different possible isotopes and I can't tell what the question wants.

c- cold periods in earth's history during which glaciers covered much of the surface.

explanation: