Choice A is the correct answer. Identify the rate and starting value.

1. Rate (Slope): $30 per month $\rightarrow 30m$.
2. Start (Intercept): $50 one-time fee $\rightarrow + 50$.
3. Equation: $C = 30m + 50$.

💡 Strategic Tip: The number associated with the variable (per month, per hour) is the slope. The one-time fee is the y-intercept.

Choice B is incorrect because it swaps the monthly rate and joining fee. Choice C is incorrect because it subtracts the joining fee. Choice D is incorrect because it adds the two numbers together as a single rate.