Choice A is the correct answer. Solve for $t$ when $C(t) = 100$.

1. Equation: $100 = 500e^{-0.15t}$.
2. Divide: $0.2 = e^{-0.15t}$.
3. Natural log: $\ln(0.2) = -0.15t$.
4. Solve: $t = \frac{\ln(0.2)}{-0.15} = \frac{-1.609}{-0.15} \approx 10.73 \approx 10.7$ minutes.

💡 Strategic Tip: For decay to reach 1/5 of original: $\ln(0.2) \approx -1.609$.

Choice B is incorrect because this is slightly too long. Choice C is incorrect because this is too short. Choice D is incorrect because this is way too long.