Which of the following observed facts about an industry are inconsistent with its being a perfectly competitive industry?
a. Different firms use different methods of production.
b. The industry's product is extensively advertised by a trade association. .
c. Individual firms devote a large fraction of their sales receipts to advertising their own product brands.
d. There are 24 firms in the industry.
e. The largest firm in the industry makes 40 percent of the sales, and the next largest firm makes 20 percent of the sales, but the products are identical, and there are 61 other firms.
f. All firms made large profits last year.