Which of the Following Statements Is True

Which of the following statements is true?

A) Economists do not know if optimal product differentiation is achieved.
B) According to the theory of monopolistic competition, the cost of product differentiation is that firms often suffer losses.
C) Methods to measure the benefits of product differentiation are well developed.
D) Product differentiation has its costs, but it does not result in any benefit.
E) Product differentiation is always desirable for consumers.