21) The petty cash can be checked at any time, even if the petty cash book is not up to date. Which of the following formulae is used for checking the petty cash? a) Petty cash vouchers + IOUs + cash held - receipt vouchers = imprest b) Petty cash vouchers - IOUs - cash held + receipts vouchers = imprest c) Imprest + petty cash vouchers = cash held - IOUs d) Imprest + cash held = petty cash vouchers