The CPA firm  found that net income had been overstated.  Which of the following errors could be the cause?
A、Failure to covert unexpired insurance to insurance expense.
B、No entry made to record purchase of land for cash on the last day of the year.
C、Failure to record payment of an account payable on the last day of the year.
D、Failure to make an adjusting entry to record revenue that had been earned but not yet billed to customers.