Which of the following is correct regarding the use of confirmation letters in an audit of financial statements:

a. A positive confirmation letter is preferred over a negative confirmation letter when the auditor, based on his past experience with the client, expects minimal to zero errors.

b. Blank confirmation letters are usually preferred when the auditor expects an overstatement error in the account balance.

c. A reply from a receivable confirmation letter received from the customer through the client is considered an invalid reply, thus another set of confirmation letter should be sent.

d. When an auditor does not receive a reply from a customer for a negative confirmation letter in a considerable period of time, the auditor should send out another set of confirmation letter.