Ex 5.3
Ex 5.5
Ex 5.7
Ex 5.8
It might be desirable to generate both diagrams, because thorough specification benefits from attacking the problem from multiple perspectives. The activity diagram focus a bit more on how the user is interacting with the system, and the sequence diagram gives a more detailed picture of how the various system componenets are interacting with one another. Doing both diagrams could help you more thoroughly consider requirements.