You need to configure the DNS resource records for incoming e-mail messages from the Internet. What should you do？（）

• A、Configure mailbox （MB） resource records in the external DNS domain. Configure each record so that it has the same refresh interval
• B、Configure mailbox （MB） resource records in the external DNS domain. Configure each record so that it has the same default Time to Live （TTL） interval
• C、Configure mail exchanger （MX） resource records in the external DNS domain. Configure each record to have the same preference and a different mail exchanger
• D、Configure mail exchanger （MX） resource records in the external DNS domain. Configure each record to have the same preference and the same mail exchanger