4. Which one of the following statements is correct?

(a) The price elasticity of the demand for salt is very low. The suppliers of salt, therefore,

have an incentive to reduce the price of salt.

(b) The demand for leisure air travel is price elastic. The suppliers of air travel, therefore,

have an incentive to raise the price of air travel.

(c) A horizontal demand curve is perfectly price elastic.

(d) The price elasticity of a vertical demand curve lies somewhere between 0.1 and 1