Possible Benefits of a Monopoly Include Which of the Following

Possible benefits of a monopoly include which of the following (choose all that apply) ?

A) a savings of fixed costs because only one firm supplies quantity demanded.
B) greater opportunities for research due to long-run positive economic profits.
C) the government is better able to ensure that it follows laws and guidelines because there is only one firm to monitor.
D) goods and services are provided at a lower price than under perfect competition because of a monopoly's decreasing average cost curve.