What is the difference between cross-sectional and longitudinal research?
a: one method is qualitative research and the other is an experiment.
b: each study requires a different amount of time involved.
c: one research method studies people and the other method studies animals.
d: no difference really exists between the two.

this would be a math

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.