The percentage change in the number of trips in central London gives you one of the numbers you need to calculate an estimate of the elasticity of demand. The other number you need is the percentage change in the price a driver pays for driving in the central city.

Part of the new cost of a car trip in central London is the fee that drivers now have to pay. In dollars, this fee is about:

a) $5

b) $20

c) $2

d) $10

e) $8