Which of the following allows monopolistically competitive firms to differentiate their products from competitors in a market?

In recent years, average household income in the United States was highest for households headed by someone aged

A) 35 to 44. B) 45 to 54. C) 55 to 64. D) 65 and over.

If the marginal cost were $18, output would be

A. 1.
B. 2.
C. 3.
D. 4.