Hi, I have a question for which the answer is provided but I don't know how it is derived, could you help to explain it step by step please?

Thank you in advance!

A monopolist faces a constant marginal cost of $1 per unit. If at the price he is charging, the price elasticity of demand for the monopolist's output is -0.5, then O the price he is charging must be $2. You Answered the price he is charging must be less than $2. O the price he is charging must exceed $2. O the monopolist must use price discrimination. Correct answer O the monopolist cannot be maximizing profits