An HR manager is interested in determining whether longer sales calls result in higher-value sales. To gain insight into this relationship, the manager divides the sales department into two "teams," and sets up an experiment. The members of the first team are encouraged to spend as much time on the phone with each customer as they are able, while the members of the second team are told that they should aim to keep calls under 7 minutes long. In this experiment, what is the independent variable? a The value of sales that are made b The amount of time salespeople spend on the phone with customers c Which team a salesperson is assigned to d That longer calls will result in higher-value sales