Bai tap when after before

1. When Don arrived, we were having coffee.
2. He sat down on a chair while I was painting it
3. The students were playing a game when the professor arrived
4. Felix phoned the fire brigade when the cooker catched fire
5. While he was walking in the mountains, Henry saw a bear
6. When the starter fried his pistol, the race began
7. I was walking home when it started to rain
8. When Margaret opened the door, the phone was ringing
9. He was sitting in the garden when a wasp stung him on the nose
10. While he was running for a bus, he collided with a lamp post
11. Vicky was having a beautiful dream when the alarm clock rang
12. When Alex saw the question, he knew the answer immediately
13. The train was waiting when we arrived at the station
14. Sarah had an electric shock when she touched the wire
15. When the campers woke, they saw the sun was shining