Question

Fossils are dated by
A. Amount of calcium
B. Radioactive carbon content
C. Association of mammals
D. Both A and B

Answer 1

There are two main methods for determining the age of fossils: relative dating and absolute dating. Absolute dating is a method used for determining the exact age of a fossil by using radiometric dating for measuring decay of isotope either the fossil or the rock associated with the fossil.

Complete answer:
Among the physical methods, the ${C^{14}}$ dating technique for the dating of organic residues is unmatched for accuracy.
The date range is $50,000$ years for a short half-life. The ${C^{14}}$ technique was developed by WF Libby $\left( {1955} \right)$). This method is based on the fact that C14 atoms are continuously generated in the atmosphere as a result of the neutron (n), proton (p) reaction induced by slow neutrons from cosmic rays in the atmospheric nitrogen cycle${N^{14}}$.
The newly formed carbon is oxidized to $^{14}C{O_2}$ and rapidly mixes with carbon dioxide in the atmosphere$\left( {^{12}C{O_2}} \right)$. A portion of the $^{14}C{O_2}$ and $^{12}C{O_2}$ in the atmosphere enters plant tissues through photosynthesis. Animals ingest this carbon by consuming plant substances.
Most of the $^{14}C{O_2}$ ends up in the ocean, where it is contained in marine carbonates. From the atmosphere, where it was born, C spreads around the world via the carbon cycle.
So, the correct answer is “Option B”.

Note:
All living things on earth are characterized by radiocarbon atoms at a constant rate (g carbon activity). The amount of $^{14}C$ present in this system is about $1 \times 10$ per ordinary carbon atom$\left( {^{12}C} \right)$. The $^{14}C$ atom follows radioactive decay, in which a neutron is converted into a proton by releasing a negatively charged beta $\left( \beta \right)$ particle called a negatron. As a result, the nucleus loses neutrons but gains protons and becomes a stable nitrogen atom.