The reason carbon dating works is that a. there is so much non-radioactive carbon dioxide in the air. b. when a plant or animal dies, it stops producing oxygen. c. plants and animals are such strong emitters of carbon-14. d. after a plant or animal dies, it stops taking in fresh carbon-14. e. none of the above

D.  There is a known constant concentration of C14 in Nature.  As we consume living things to survive our bodies (made of carbon) stop replenishing our body's carbon (we stop eating) and start to decay.  Since we know the 1/2 life of C14 and the ratio of C14 to normal C12 we can determine fairly accurately how long ago a thing stopped consuming carbon (e.g. when it died)