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See explanation below.

Explanation:

An oligopoly refers to a market structure in which a few firms dominate. This means that, it is a market or industry which is dominated by a small group of large sellers. When a market is shared between a few firms, it is highly concentrated. In this type of market, only a few firms account for a large output of sales.

From the definition above, we can therefore explain the statement presented above by saying, an oligopoly cannot be defined by virtue of the number of firms in an industry alone, but rather by the level of output sales that are achieved by a very few number of large firms or corporations.