Fill in for, by or on when necessary.
1    I prefer travelling ... train to travelling ... air. I get airsick, actually.
2    — Can I speak to James, please?
—    Sorry, he is not in. He has left ... Paris for a couple of weeks.
3    It's already very late. If you don' leave ... immediately, there won't be any buses and you will have to go ... foot.
4    — I've heard you are going to leave ... your job. Is this true?
—    Yes, I have to do it because I'm leaving ... St Petersburg ... Samara.