Choice A is the correct answer. Calculate doublings over 2 hours.

1. Periods: 2 hours = 120 minutes, so $\frac{120}{20} = 6$ doublings.
2. Formula: $N(t) = 100(2)^{t/20}$ where $t = 120$ minutes.
3. Calculate: $N(120) = 100(2)^6 = 100(64) = 6,400$.

💡 Strategic Tip: 6 doublings = multiply by $2^6 = 64$.

Choice B is incorrect because this assumes linear growth. Choice C is incorrect because this is after only 3 doublings. Choice D is incorrect because this is after 5 doublings.