Which of the statements is/are true for data validation?

Which of the statements is/are true for data validation?
A . Data Validation can enforce data integrity as well as make a field required in the page layout
B . Data Validation can prevent duplication of records
C . Data Validation run on the client machine after the user clicks the Save button
D . Data Validation rules are not enforced when using API & import operations
E . Validation rules can be activated / deactivated by the user

Answer: A,B