A radioactive sample has mass 18 mg after 10 years. Its rate of change satisfies dtdy=−0.07y, where t is in years. Which of the following statements about the initial mass y0 at t=0 is most accurate?
a) y0<18 mg because the decay constant is positive.
b) y0=18 mg because the mass is independent of time during exponential decay.
c) y0>18 mg because the decay constant is negative and time is positive.
d) y0<18 mg because an exponent with a negative coefficient causes the quantity to increase over time.