The wet weather will continue tomorrow, when a cold front________ to arrive.

Research shows that humans ______ from selfish to unselfish behavior when they are watched.

 The meeting was put off because we ________ a meeting without John.

The watch was very good, and he    20 percent down for it.

(2013·陕西宝鸡模拟)After Queen Elizabeth officially opened the 2012 Summer Olympics, London became the first city ________ three Olympic Games in history.