Rewrite the sentences in the passive where possible

9) We were discussing the book when the author arrived
10) Students were checking the exercisers while the bell was ringing
11) Yesterday I asked my teacher to help me
12) Teacher gave me a good mark
13) The boys will paint the roof of the house
14) Tom gave Nick a book for his birthday
15) A boy showed her the way
16) Our mother tells us stories every evening
17) The elephant broke the branch of the tree
18) Helen was washing the dishes and Betty was listening to the music
19) I have just watered the plants
20) They have been waiting for Tom since morning