Which of the Following Would Least Likely Be a Barrier

Which of the following would least likely be a barrier to entry into a monopoly?

A) Economies of scale enjoyed by a large firm.
B) Tariffs on foreign goods are eliminated.
C) A company is the sole inventor of what it produces and no one else can make a good substitute.
D) Government restrictions such as license requirements are enacted.
E) A company is the only owner of an essential resource needed to produce its product.