Indicate the objective of each of the following controls.
a. Canceling paid vouchers by perforating them at the time of payment
b. Simultaneously reconciling all bank accounts
c. Using pre-numbered checks and carefully accounting for used and unused checks
d. Maintaining a record of numbers of all stock certificates and bonds
e. Periodically comparing personnel department rosters with payroll registers
f. Having checks mailed by people other than those causing them to be drawn