Which of the Following Is Ordinarily Considered an "Extended Procedure

Which of the following is ordinarily considered an "extended procedure" during the independent audit of financial statements?

A) Send positive confirmations on recorded customer accounts receivable balances.
B) Perform physical observation and test count during the client's inventory taking.
C) Measure the time lag between the date of recording cash receipts in the books to the date of deposit credit in the bank.
D) Conduct interviews with the client's sales billing personnel to learn about sales recording control activities.