As a Measure of Competition in an Industry, Concentration Ratios

As a measure of competition in an industry, concentration ratios have several flaws.One of these flaws is that concentration ratios

A) assume that all industries have low barriers to entry.
B) assume that a ratio less than 40 percent means an industry is perfectly competitive.
C) assume there are only four firms in an industry.
D) are calculated for the national market, even though competition in some industries is mainly local.