Why was the cleveland massacre significant in the formation of standard oil?

Monopoly in the Cleveland oil market after the Massacre, is the right answer.

Explanation:

Rockefeller who founded the Standard Oil planned the Cleveland massacre to ambush the oil market in the United States. Throughout the decade of 1870's, a number of oil refining businesses competed with each other in the regions of Pennsylvania. The increasing competition among companies made the prices of black oil volatile. Moreover, some of the companies were prompt to present exorbitant prices just to obtain an edge on the race.

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