Four samples each contain a single radioactive isotope. Sample A has 1 mol of matter and an activity of 100 Bq. Sample B has 10 mol and 100 Bq, sample C has 100 mol and 100 Bq, and sample D has 100 mol and 1000 Bq. Rank in order, from largest to smallest, the half-lives of these four isotopes. Explain.