Which of the following would be considered a ?dual-purpose test? in an audit?

Sending negative confirmations of receivables to customers with small recorded balances and positive confirmations to accounts with large balances

Vouching sales invoices for authorization and tracing the amount of each invoice to the sales journal

Comparing the amount determined according to the amortization method used by the client to the amount reported in the financial statements

Reviewing the client's inventory-taking instructions and observing whether the instructions are Followed