The CPA firm auditing Mason Street Recording Studios found that total stockholders' equity was understated and liabilities were overstated. Which of the following errors could have been the cause?

a. Making the adjustment entry for depreciation expense twice.

b. Failure to record interest accrued on a note payable.

c. Failure to make the adjusting entry to record revenue that had been earned but not yet billed to clients.

d. Failure to record the earned portion of fees received in advance.