On average, a call center receives calls from customers every 7 minutes, and it takes 15 minutes to finish a call. The system assumes Poisson arrivals and exponential service time. The manager's goal is to limit the average customer waiting time to 2 minutes. What is the minimum number of customer services the call center needs to have to achieve this goal? (Hint: compare the waiting time of all four options.)

*Please consider at least two decimals for each of your calculations.