Homework for week 2: assignments/session02

Hi, 
The following is my week2 homework. I don't know how to remove early commits from homework 1 for this pull request. It seems that it includes all change sets. Any comments on how to select which changeset to submit?
commit: b4cb283 for required tasks.
commit: 8558c75 for 3 optional tasks I selected.
Thanks,
Gary