By 1878, the Standard Oil Company, owned by John D. Rockefeller, had bought out most of its business rivals and controlled 90% of the petroleum refineries in the United States. Which of the following was a likely effect of Standard Oil's business practices?
a. The company set limits on its prices
b. The company increased oil prices
c. Competition in the oil market flourished
d. Standard Oil increased its efforts to attract needed customers
e. The federal government offered a subsidy to make the company more competitive abroad​