Choice A is the correct answer. Find when concentration halves.

1. Half: $C(t) = 100$ (half of 200).
2. Equation: $100 = 200e^{-0.2t}$.
3. Divide: $0.5 = e^{-0.2t}$.
4. Natural log: $\ln(0.5) = -0.2t$.
5. Solve: $t = \frac{\ln(0.5)}{-0.2} = \frac{-0.693}{-0.2} \approx 3.47 \approx 3.5$ hours.

💡 Strategic Tip: Half-life formula: $t_{1/2} = \frac{\ln(2)}{k}$ for decay $e^{-kt}$.

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