What was the effect of Rockefeller's affiliation with the South Improvement Company in 1872, and why did it cause his competitors in the oil industry to call it a "conspiracy"?

The south improvement company in 1872 caused rockefeller able to monopolized around 90% of the oil market in North America.

The south improvement company largely increased the number of oil and railroad companies in southern states. Because of the increase in numbers, the market for oil and railroad companies became saturated. This lead many of his competitors to bankruptcy, which is why the competitors called it a conspiracy.