Concept: Radioactive decay reduces the number of atoms by half after every half-life period.Explanation: Start with 1000 atoms.After one half-life of 20 seconds, 500 atoms remain.After two half-lives, 40 seconds, 250 atoms remain.After three half-lives, only 125 atoms remain.Each half-life is 20 seconds.Total time taken is 3×20=60 seconds.Answer: D. 60 seconds.