Why are fossil fuels supplies limited

The coal, oil and natural gas we are consuming now are the remnants of plants, trees and animals that were living hundreds of millions of years ago. There is no way to replace them, as conditions on Earth are much different now.

Explanation: Mines and wells are being depleted much faster than they were a hundred years ago when demand was much less-- fewer people, fewer cars, fewer planes. . . and people believed there was an endless supply. Many of those mines and oil wells are now empty.

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.

bacterium is not correct