Which of the following statements is TRUE about primary keys and foreign keys? A. primary keys cannot be null B. data values of a foreign key always exist in the corresponding table C. foreign keys could be null Multiple Choice A, B A, C B, C A, B, C Which of the following statements is TRUE about primary keys and foreign keys? A. primary keys cannot be null B. data values of a foreign key always exist in the corresponding table C. foreign keys could be null Multiple Choice A, B A, C B, C A, B, C Which of the following statements is TRUE about primary keys and foreign keys? A. primary keys cannot be null B. data values of a foreign key always exist in the corresponding table C. foreign keys could be null Multiple Choice A, B A, C B, C A, B, C