应该是最基本的，本人是新手，这学期才学的．

Suppose you are in charge of a toll bridge that is essentially cost-free. The demand for bridge crossings, Q, is given by P=12-2Q,

a) What is the socially optimal price for crossing the bridge?How many people will cross the bridge at that price?

b) if you were a monopolist, what price would you charge?

c) what is the elasticity of demand at the monopoly price?