Radioactive Decay: A sample containing 4.0 × 1018 atoms of a radioactive isotope decays with a half-life of 2.3 years. How many undecayed atoms are left after 3.7 years?
A. 2.5 × 1018
B. 1.7 × 1018
C. 1.3 × 1018
D. 1.1 × 1018
Answer: C
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