The correct answer is: B. Equal to one
The elasticity of demand is a measure of how responsive consumers are to changes in price. It is calculated by dividing the percentage change in quantity demanded by the percentage change in price.
A straight demand line has a constant elasticity of demand. This means that the percentage change in quantity demanded is always equal to the percentage change in price.
The elasticity of demand at the midpoint on a straight demand line is equal to one. This is because the percentage change in quantity demanded is equal to the percentage change in price at the midpoint.
Option A is incorrect because the elasticity of demand at the midpoint on a straight demand line is not equal to zero.
Option C is incorrect because the elasticity of demand at the midpoint on a straight demand line is not greater than one.
Option D is incorrect because the elasticity of demand at the midpoint on a straight demand line is not less than one.