函数$f\left(x\right)=\sin x\sin \left(x+\frac{\mathrm{\pi }}{2}\right)$的最小正周期为(      )

    Hundreds of years ago, news was carried from place to place by people on foot or by horse. It took days, weeks and sometimes months for people to receive news. Now it is possible to send words and pictures around the world in seconds(秒). Billions of people learn about news stories of their own country and all over the world every day, either by watching TV or reading newspapers. Newspapers have been an important part of everyday life since the 18th century. Many countries have hundreds of different newspapers. How do newspaper editors(编辑) decide which news stories to print (印刷)? Why do they print some stories and not others? What makes a good newspaper story?

    Firstly, it is important to report new stories. TV stations can report news much faster than newspapers. Yet, newspapers give more about the same story. They may also look at the story in another way, or they may print completely (完全地) different stories to those on TV.

    Secondly, a news story has to be interesting and unusual. People don't want to read stories about everyday life As a result, many stories are about some kind of danger and seem to be "bad" news. For example, newspapers never print stories about planes landing safely, instead they print stories about plane accidents.

    Another factor(因素) is also very important in many news stories. Many people are interested in news in foreign countries, but more prefer to read stories about people, places and events (事件) in their own country. So the stories on the front page in Chinese newspapers are usually very different from the ones in British, French and American newspapers.

气孔是植物体与外界进行气体交换的“窗口”．气孔的开闭受（  ）控制．

我来猜。

七丢一横变成撇。

             谜底是____。