In the third problem, first two small problems are easy. The theory  used is just the basic definition of the limitation. In (c) problem, we should rethink about the definition  and figure out what conditions do we have and what is needed to prove the (c) problem. Also, when you do this problem, you should not trap in the answer of (a) and (b). The condition in the (a) should be linked with the condition of (b). Besides, the transfer is also important in this problem.