How Come an Animal's Carbon-14 Levels Do Not Start to Decrease

Multiple Choice

How come an animal's carbon-14 levels do not start to decrease until it dies?

A) because it constantly replenishes its carbon-14 supply by eating plants with carbon-14 in them
B) because it constantly replenishes its carbon-14 supply by breathing air with carbon-14 in it
C) because it is constantly being bombarded with cosmic rays
D) carbon-14 only starts to decay after the animal dies
E) The carbon-14 levels do not decrease because the carbon-13 in its skin protects it from cosmic rays.