1. To drive a London taxi, drivers must spend beforehand two years learning the London streets.

The value of their time spent doing this is:

a. zero marginal cost

b. infinite

c. zero, since they are not paid

d. their opportunity cost

2.Having spent that time learning, the cost of that time is:

a. their variable cost

b. their marginal costs

c. their sunk cost

d. their average cost