4. A table named E-commerce has three attributes, which are {Roll_no, Name, and Registration no. In this table, we have identified superkeys as follows: {Roll_no} {Registration_no} {Roll_no, Registration_no} {Roll_no, Name} {Name, Registration no) {Roll_no, Name, Registration_no} Based on the examples, which one is BEST selected to be the candidate keys and primary key? A. candidate keys = {Roll_no) and {Registration_no}; primary key = {Roll_no} B. candidate keys = {Roll_no, Registration_no} and {Roll_no, Name primary key = {Registration_no) C. candidate keys = {Roll_no} and {Registration_no} primary key {Roll no and {Registration_no} D. candidate keys = {Roll_no, Name, Registration_no} primary key = {Registration_no} 5. An attribute of age can only accept two characters of integer values. Select the MOST suitable constraints to match with the following statement. A. Referential constraints B. Domain constraints C. Entity constraints D. General constraints 6. Several disadvantages of Database approach are listed below, EXCEPT? A. Complexity B. Cost of conversion C. Sharing of Data D. Cost of DBMSs