Many scientists have believed that there may be a _________of life on the Mar．

We have worked out the plan and now we must put it into＿＿＿．

—I’m sorry I stepped outside for a smoke. I was very tired. —There is no______ for this while you are on duty. 

He is an old doctor with a lot of ________. But he has never travelled by air before.Taking a plane was quite ________ experience for him this time.

 ---What do you think of the dinner in Mr Brown's?---Oh, great! We have never had a better one. It's a dinner of twenty ____.