Answer is: A The half-life of reactants in a nuclear decay reaction remains constant over time, but the mass of reactants that are consumed decreases.
1) For example, the half-life for the radioactive decay of U-238 is 4.5 billion years and is independent of initial concentration.
2) Another example, half-life is the time required for a quantity to reduce to half its initial value.
Initial mass of sample: 30 grams.
After first half-life (60 days): 50% · 30 g ÷ 100% = 15 g.
After second half-life (120 days): 0.5 · 15 g = 7.5 g.
