18. A set S cRis said to be a discrete set if S' = ộ. A set S CR is said to be an isolated set if Sns'= o (i.e., if each point of S is an isolated point). i) Prove that every discrete set is an isolated set, but not conversely. (i) Give an example of an infinite discrete set S CR. (ii) Give an example of a bounded discrete set S C R. iv) Can there be an infinite bounded discrete set S CR?