A marketing analyst conducts an experiment to investigate the effect of offering airline miles on the amount charged by a customer on a credit card. Customers were offered one of four airline mileage offers (no miles, standard miles, double miles, or "use anywhere miles"). The analyst is also interested in whether sending the offer in an envelope with a prominent airline logo will have an effect, so the offer is sent out in a standard envelope or in an envelope with a prominent airline logo on it. a. Identify the response variable. b. Identify the explanatory variable(s). c. How many treatments are there?