The general format of Internet host domain name is: www .. (user name). (second level domain name >; . (first-level domain name). A first-level domain name is also called a top-level domain name. Except abroad, every country or region in the world has a unique first-level domain name. For example, cn stands for China, jp stands for Japan, uk stands for Britain and tw stands for China and Taiwan Province Province. The second-level domain name is generally the industry to which the host belongs. For example, edu is an educational institution, corrl is a commercial institution, net is the main network support center, gov is a government department, mil is a military organization, int is an international organization, ac is a scientific research institution, and org is a non-profit organization. So choose BCD.