You are the network administrator for The network consists of a single DNS domain named. You replace a UNIX server with a Windows Server 2003 computer named TestKing1. TestKing1 is the DNS server and start authority (SOA) for testking.com. A UNIX server named TestKing2 is the mail server for testking.com. You receive reports that Internet users cannot send e-mail to the testking.com domain. The host addresses are shown in the following window. You need to ensure that Internet users can send e-mail to the testking.com domain. What should you do?（）
- A Add an _smtp service locator (SRV) DNS record for TestKing2.
- B Add a mail exchange (MX) DNS record for TestKing2.
- C Add an alias (CNAME) record for mail.testking.com.
- D Enable the SMTP service on TestKing1.