11. Things have changed a lot…………….our previous meeting.
12. It's been raining…………….four o'clock. 
13. I'm sure he's been watching us …………….we came in.
14. He'd been here…………….over an hour when we arrived.
15. I've been trying to fix this computer…………….early this morning.
16. He hasn't had anything to eat…………….nearly a week.
17. It's three years…………….he started learning German.
18. He's been collecting stamps…………….the past twenty years.