Which of the Following Describes a Correct Relation Between Price $Q > a / 2 b$

Multiple Choice

Which of the following describes a correct relation between price elasticity of demand and a monopolist's marginal revenue when inverse demand is linear, P = a-bQ?

A) Demand is elastic when $Q > a / 2 b$ .
B) Demand is inelastic when $Q > a / b$ .
C) Demand is unit elastic when $P = a / 2 b$ .
D) Demand is elastic when $Q < a / 2 b$ .