I. 1. at

   2. about

   3. a set of

   4. Come on

II. 1. You’re welcome / Not at all

    2. Excuse me

    3. everywhere

    4. his; hers

    5. under

    6. sending an e-mail 

III. 1. mine

     2. in

     3. its

     4. on

     5. theirs

     6. under

IV. One possible version:

Hello, everybody! This is my room. It is light blue. There is a bed and a desk in it. The bed is big and comfortable. A computer is on the desk. I can play computer games after I finish my homework. On the desk, you can see some colorful flowers, too. I often clean my room so it is always clean and tidy. The room is a good place for me to study and relax. I like my room very much.