Could someone explain why the first answer would be incorrect?
I felt it was a correct answer based on SQL 2005 BOL:
SQL Server 2005 supports the ability to impersonate another principal either explicitly by using the stand-alone EXECUTE AS statement, or implicitly by using the EXECUTE AS clause on modules. The stand-alone EXECUTE AS statement can be used to impersonate server-level principals, or logins, by using the EXECUTE AS LOGIN statement. The stand-alone EXECUTE AS statement can also be used to impersonate database level principals, or users, by using the EXECUTE AS USER statement.
Implicit impersonations that are performed through the EXECUTE AS clause on modules impersonate the specified user or login at the database or server level. This impersonation depends on whether the module is a database-level module, such as a stored procedure or function, or a server-level module, such as a server-level trigger.