Classify the following as either a revenue or a capital expenditure.
a. Paid $40,000 cash to replace a motor on equipment that extends its useful life by four years.
b. Paid $200 cash per truck for the cost of their annual tune-ups.
c. Paid $175 for the monthly cost of replacement filters on an air-conditioning system.
d. Completed an addition to a building for $225,000 cash.