Weather Door manufactures and sells storm doors. If the price of a storm door is $287, then the company sells 142 doors per week, and the elasticity of demand is -1.6. Assume that the demand function is differentiable and that the only time the company sells 142 doors per week is when the price of a door is $287. 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.] o a) The marginal revenue is negative when Weather Door sells 142 doors per week. o b) The marginal profit is positive when Weather Door sells 142 doors per week. O c) The marginal profit is negative when Weather Door sells 142 doors per week. o d) The marginal revenue is positive when Weather Door sells 142 doors per week