A Normally Trustworthy Cashier Who Opened a Company's Mail Discovered

A normally trustworthy cashier who opened a company's mail discovered a customer paid off his account in cash ($500) . The cashier was working alone at the time and pocketed the cash. She couldn't resist the temptation because she needed to buy some expensive medicine for her infant daughter and the company's insurance policy wouldn't cover the expense. Later, the cashier accessed the company's accounting system and entered a credit memo for that customer in the amount of $500 (so the customer wouldn't receive another bill and complain that he had already paid it) . Which of the following controls would have been LEAST effective in PREVENTING this crime?

A) Require the credit manager's password to be used to enter credit memos into the system
B) Require more than one employee to be present when opening mail
C) Restrict physical access to the company's system so only employees who have no custody or authorization functions can access it
D) Require the cashier to be bonded
E) Monitor the cashier workstation with video cameras and make employees aware their actions are being monitored in that way