a.the bottom (smallest) 20 firms control 1 percent of the industry output.
b.the top (largest) 20 firms control 99 percent of industry output.
c.the bottom (smallest) 20 percent of firms control 99 percent of industry output.
d.99 firms control 20 percent of industry output.
e.none of the above.
Concentration ratios tell you how much of an industry is controlled by the biggest firms; it indicates how oligopolistic or fragmented a market is, and hence how easy it may be for new entrants to break in or smaller firms to increase market share. In this case, the answer must be b: the largest 20 firms control 99% of industry output.
It depends on the context. Not enough information.