Answer:
a. We were laughing at Sonia's joke when the teacher was entering the class.
b. While Karen was waiting for the bus, she saw her best friend.
c. They weren't allowing us to have our mobile phones while we were studying in the library.
d. Danny was standing on the chair when his mum appeared at the door.
e. The teacher was collecting the paper while I was answering the last question.
f. She didn't write her essay last night because her brothers made too much noise.
