题干

一根通电直导线在某个空间没有受到安培力的作用,那么(   )

A:这个空间一定没有磁场

B:这个空间可能有方向与电流方向平行的磁场

C:这个空间可能有方向与电流方向垂直的磁场

D:以上三种说法都不对

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答案(点此获取答案解析)

B

同类题4

      Without proper planning, tourism can cause problems. For example, too many tourists can crowd public places that are also enjoyed by the local people of a country. If tourists create too much traffic, the local people become unhappy. They begin to dislike tourists and to treat them impolitely. They forget how much tourism can help the country's economy(经济). It is important to think about the people of a destination country and how tourism affects them. Tourism should help a country keep the customs and beauty that attract tourists. Tourism should also advance(推进) the well-being (health and happiness) of local people.
      Too much tourism can be a problem. If tourism grows too quickly, people must leave other jobs to work in the tourism industry. This means that other parts of the country's economy can suffer.
       On the other hand, if there is not enough tourism, people can lose jobs. Businesses can also lose money. It costs a lot of money to build large hotels, airports, air terminals, first-class roads and other support facilities needed by tourist attractions. For example, a major international-class tourism hotel can cost as much as 50 thousand dollars per room to build. If this room is not used most of the time, the owners of the hotel will lose money.
      Building a hotel is just a beginning. There must be many support facilities as well, including roads to get to the hotel, electricity, sewers to handle waste, and water. All of these support facilities cost money. If they are not used because there are not enough tourists, jobs and money are lost.