Why my answer in red is incorrect ? , and what is the correct answer

The market for plumbing services in a city can be characterized by the model of monopolistic competition. Suppose that the market is initially in long-run equilibrium, and then there is an increase in demand for plumbing services. We expect that in the long run: Select one: O a. firms will leave the plumbing market. O b. there will be a short-run increase in the number of firms, but then the number will return to the original level. X O c. new firms will enter the plumbing market. O d. firms will shut down, but they will not leave the industry. The correct answer is: new firms will enter the plumbing market