Explain your answer please. Thank you!

Adam Signoff-On, CPA, was auditing Defense Industries, Incorporated Signoff-On sent positive accounts receivable confirmations to a number of Defense's government customers. He received a number of returned confirmations marked "We do not confirm balances because we are on a voucher system." Required: a. List alternative procedures that Signoff-On might use to ensure the validity of these accounts. b. Assuming all the procedures you list are viable options for the Defense audit, which procedure do you believe provides the highestquality evidence and why