Which of the following would NOT be a contributing factor for the ending A/R to be significantly different from the beginning A/R in a given period?

a. A write off some receivables that were deemed uncollectible.

b. Billing personnel have redirected their attention to a project and have not been able to call customers about payment reminders.

c. The company refunded several customers who paid cash for their services.

d. Credit sales experienced a 20% increase during the period.