Weather Door manufactures and sells storm doors. If the price of a storm door is $212, then the company sells 86 doors per week, and the elasticity of demand is -0.54. Assume that the demand function is differentiable and that the only time the company sells 86 doors per week is when the price of a door is $212. 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 profit is positive when Weather Door sells 86 doors per week. O b) The marginal revenue is positive when Weather Door sells $6 doors per week, O o The marginal profit is negative when Weather Door sells 86 doors per week. O d) The marginal revenue is negative when Weather Door sells $6 doors per week