The tomato juice left some _____ on a customer’s skirt, which made the waitress nervous.

一Tell us, Mark ,what is the ____ of the American Civil War?一 Well, is it because the black slaves wanted freedom?

The members of the club wouldn't run a _____ in entrusting（委托） the organization to an unreliable person.

But the unpleasant feeling of coldness disappeared at the _______ of his students pouring out of their classroom to give him a warm, wordless welcome.

In a time of social reform, people’s state of mind tends to keep _______ with the rapid changes of society.