DATABASE 4. a) An advantage of mandatory access control over discretionary access control is that it provides better control over the flow of information. How is this flow control achieved? b) Why do you get a poorer flow control with the method discretionary access control? c) The method includes filtering and polyinstanting of tables. What do these terms mean? Explain with the help of an example d) Explain what physical data independence is and why it is so important to achieve this.