Your Program Uses a Map Structure to Store a Number

Your program uses a Map structure to store a number of user ids and corresponding email addresses. Although each user must have a unique id, two or more users can share the same email address. Select an appropriate expression to complete the method below, which adds a new id and email address to the map only if the id is not already in use. If the id is already in use, an error message is printed. public static void addUserID(Map<String, String> users, String id, String email)
{
String currentEmail = users.get(id) ;
If (___________________)
{
Users.put(id, email) ;
}
Else
{
System.out.println(id + " is already in use.") ;
}
}

A) currentEmail.equals(email)
B) !currentEmail.equals(email)
C) currentEmail == null
D) currentEmail != null