Answer:
He bought another businesses and products in order to increase or expand his business further. Refining oil was the business that made the standard oil a horizontal monopoly.
The government was though against the illegal monopoly. He bought other businesses which decreased the cost of production. This helped him increase his profits and business to such an extent that there was no other competitor in the market.
It owned 90% of the oil refineries in United States. Hence explaining the monopoly.