According to the Center of Disease Control and Prevention (CDC) (疾病控制中心), if you don’t often wash your hands, you can pick up viruses (病毒)from many places and make yourself ill. You’re at risk every time you touch your eyes, nose or mouth. In fact, one of the most common ways people catch colds is by rubbing their noses or their eyes when there are cold viruses on their hands. If you don’t often wash yours hands, specially when you’re sick, you may spread the viruses directly to others. You should wash your hands in the following situations:
After using the bathroom
After blowing your nose or coughing
Before eating, serving or preparing food
After touching pets or other animals
After visiting anyone who is sick
When your hands are dirty
There is a right way to wash your hands. Follow these simple steps to keep your hands clean.
Use warm water (not cold or hot)
Wash for 15 seconds
Rinse (冲洗) well under warm water and dry them with a clean towel.
Information Card

    Black Friday

    Thanksgiving is always on a Thursday.The day after it has been known as Black Friday. It is the start of the holiday shopping season,and it has been the busiest shoppingday of the year in the US since 2005.

    Most stores offer big sales on Black Friday.They open their doors quite early in the morning.They try to attract shoppers with big discounts(打折).Some items(商品) like TVs and clothes are much cheaper than usual.Stores may even lose money on these items.They hope that shoppers will buy gifts for other people while they are in the store.However,there are problems.The biggest one is that there are not enough low—priced items. Each store may only have a few so that not everyone can get one.

    The situation makes people worried.Some Black Friday events have been crazy. There have been fights over toys among shoppers. Some workers have even been hurt by large crowds. But most Black Friday events are safe and fun..So where does the name “Black Friday”come from? It was firstused in Philadelphia in the 1950s.

    Now people all over the country take part in Black Friday events.It is even spreading to other parts of the world.Stores have held Black Friday events in the UK，Australia ，and Brazil since 2012. In Mexico，stores offer a weekend of discounts every year. They call it “El Buen Fin”,which means“the good weekend ”in Spanish. I guess people everywhere like to buy things on sale.

下列线圈中不能产生交变电流的是(  )

人耳的听觉范围是____至____，频率高于____的声音叫做超声波，频率低于____的声音叫做次声．