Q. Which of the following can be addressed by enforcing a referential integrity constraint?

• (A) all phone numbers must include the area code
• (B) certain fields are required (such as the email address, or phone number) before the record is accepted
• (C) information on the customer must be known before anything can be sold to that customer
• (D) then entering an order quantity, the user must input a number and not some text (i.e., 12 rather than ‘a dozen’)