Indicate which of the following errors, each considered individually, would cause the trial balance totals to be unequal:
a. A fee of $2,350 earned and due from a client was not debited to Accounts Receivable or credited to a revenue account, because the cash had not been received.
b. A payment of $1,500 to a creditor was posted as a debit of $1,500 to Accounts Payable and a debit of $1,500 to Cash.
c. A payment of $6,000 for equipment purchased was posted as a debit of $600 to Equipment and a credit of $600 to Cash.
d. Payment of a cash withdrawal of $12,000 was journalized and posted as a debit of $21,000 to Salary Expense and a credit of $12,000 to Cash.
e. A receipt of $750 from an account receivable was journalized and posted as a debit of $750 to Cash and a credit of $750 to Fees Earned.