relevant geographic market

The proper question to be asked * * * is not where the parties to the merger do business or even where they compete, but where, within the area of competitive overlap, the effect of the merger on competition will be direct and immediate. * * * This depends upon 'the geographic structure of supplier-customer relations.' * * * the 'area of effective competition in the known line of commerce must be charted by careful selection of the market area in which the seller operates, and to which the purchaser can practicably turn for supplies.