A、Design a trigger that verifies that the first names and surnames are not empty strings.

B、Design a check constraint that verifies that the first names and surnames are not empty strings.

C、Design a check constraint that uses a CLR user-defined function to verify that either the phone number is a null value or the format of the phone number is valid.

D、Design a trigger that uses a CLR user-defined function to verify that either the phone number is a null value or the format of the phone number is valid.

E、Design a check constraint that ensures that the phone number cannot be changed from a valid format to a null value or to an invalid format.

F、Design a trigger that ensures that the phone number cannot be changed from a valid format to a null value.