1. Carbon dating is useful only for determining the age of objects less than about _____ years old.
(a) 4.5 million.
(b) 1.2 million.
(c) 600,000.
(d) 60,000.
(e) 6000.
2. Radon has a half-life of about 1600 years. The Earth is several billion years old, so why do we still find radon on this planet?
(a) Ice-age temperatures preserved some of it.
(b) Heavier unstable isotopes decay into it.
(c) It is created in lightning strikes.
(d) It is replenished by cosmic rays.
(e) Its half-life has increased over time.
(f) Its half-life has decreased over time.