Which two statements are true regarding constraints?

Which two statements are true regarding constraints? (Choose two.)

A.
A columns with the UNIQUE constraint can contain NULL values

B.
A constraint can be disabled even if the constraint column contains data

C.
A constraint is enforced only for the INSERT operation on a table

D.
A foreign key cannot contain NULL values

E.
All constraints can be defined at the column level as well as the table level