Ahuman activity that could significantly decrease
the amount of carbon dioxide in the air is
(1) increasing the use of fossil fuel
(2) controlling insect pests that eat stored grain
(3) burning garbage and trash to generate
electricity
(4) preserving and expanding forest habitats that
shelter wildlife

youre right there are

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.