Which of the following situations does not require an adjusting entry at the end of May?

a. On May 1, Bolton Industries paid rent for six months on its office building.

b. On May 1, Bolton Industries began delivery service to a client who will pay at the end of a three-month period.

c. At the end of May, Bolton Industries pays the custodian for May office cleaning services.

d. On May 1, Bolton Industries purchased delivery equipment with an estimated useful life of six years.