Firm X Is a Single Seller of Good X

Multiple Choice

Firm X is a single seller of good X. There are, however, two substitutes for good X. Given this,

A) firm X cannot be a monopolist because the theory of monopoly assumes there are no substitutes for the good the single seller sells.
B) firm X may be a monopolist because the two substitutes may be close substitutes.
C) firm X cannot be a monopolist because if substitutes exist for the good it produces, its demand curve is horizontal but monopolists face downward-sloping demand curves.
D) none of the above