According to my knowledge the above question relates to Chapter 4 ( Relational Algebra ).
However this chapter is excluded in our preparation and also not mentioned in the Tutorial letter.
Anyone in disagreement ?
There is a paragraph containing that gives the answer in the related material. I remember thinking the same thing as you, but then on careful re-read of a paragraph in search of something else, there the answer was...
Happy hunting (I would give a clue if I could remember where exactly I did find it)