If the toothpaste market is monopolistically competitive, product differentiation may take place in what way?
a. the production of different varieties of toothpaste, including those with whitening agents, differentiation in the locations where certain toothpastes are available, and quality differences among the various brands
b. differentiation in the locations where certain toothpastes are available
c. the production of different varieties of toothpaste, including those with whitening agents
d. quality differences among the various brands
