Roland Incorporated manufactures and sells portable hair dryers. If the price of a hair dryer is $ 194.00, then the company sells 138 hair dryers per week, and the elasticity of demand is -1.9. Assume that the demand function is differentiable and that the only time the company sells exactly 138 hair dryers per week is when the price of a hair dryer is $194.00. Given this situation, which of the following do you know MUST be true? [Hint: Recall that marginal revenue is the derivative of revenue with respect to quantity.] The marginal revenue is negative when Roland Sells 138 driers per week. The marginal revenue is positive when Roland sells 138 driers per week. The marginal profit is negative when Roland sells 138 driers per week. The marginal profit is positive when Roland sells 138 driers per week.