Choice A is the correct answer. Calculate the ratio between periods.

1. Period 1: $\frac{20,000}{5,000} = 4$.
2. Period 2: $\frac{80,000}{20,000} = 4$.
3. Growth Factor: Every 2 months, subscribers multiply by 4.

💡 Strategic Tip: Consistent ratio = exponential growth factor.

Choice B is incorrect because this would only give 5,000 → 10,000 → 20,000. Choice C is incorrect because this is the difference (linear), not ratio. Choice D is incorrect because the ratio is 4, not 5.