Why is carbon-14 used for carbon dating?
A. Carbon-14 is formed from cosmic rays containing neutrons so it is easy to determine which carbon is natural to Earth and which is carbon-14.
OB. It is the only radioactive element not harmful to living organisms.
OC. Carbon-14 has a half-life of millions of years so it can date extremely old remains.
D. The proportion of carbon-14 in living organisms stays constant until they die and then it begins decreasing allowing scientists to calculate approximate time of death.