关于地磁场的成因，科学家们曾提出如下假设：地球自身带有某种电荷，地球自转的环形电流产生了地磁场．则该假设中地球带电的种类和所形成的环形电流的方向分别是(磁子午线指地球磁场N极与S极在地球表面的连线)（ ）
A．正电荷，由东向西垂直磁子午线
B．正电荷，由南向北沿磁子午线
C．负电荷，由东向西垂直磁子午线
D．负电荷，由西向东垂直磁子午线
答案：C
由题意知，地磁体的N极朝南，根据安培定则，大拇指指向地磁体的N极，则四指的绕向即为电流的方向，即安培假设中的电流方向应该是由东向西垂直磁子午线.
| 阅读理解。 |
| Think of life in a desert: no food or water, only strong sun and heavy wind full of sand. You'd die of thirst while waiting for a passing plane that never comes. But don't dismiss the thought as a nightmare (噩梦). Your hometown could become a desert if nothing is done to prevent it. Deserts are eating land at high speed. In China, desertification (沙漠化) has affected 2,622,000 square kilometres, equal to 14 Guangdong Provinces, mostly in the North. Deserts are advancing at a pace of 2,460 kilometres per year. According to the UN, one-third of the planet's land surface, with one billion people, has become desert. Africa, Asia and Latin America have the driest land in danger of desertification. Recognizing the threat (威胁) to human beings, the United Nations has named 2006 the International Year of Deserts and Desertification. It wants to increase public awareness (关注) of advancing deserts. Activities include a film festival in Rome with films on life in unproductive (贫瘠的) lands. Meetings will discuss international cooperation, such as planting trees and changing agricultural practices. Desertification takes place where rain is little and the climate hot. But human activities are the biggest cause. Land turns into desert when the soil is over-farmed and too many animals eat up plants. To make money, people also cut down trees that hold the soil. Desertification brings serious problems. From 1991 to 2000, droughts (干旱) killed over 280,000 people worldwide. When land becomes useless, there's a big loss in food production. Dust from deserts is blown into cities around the world. Each spring, Beijing is at risk of yellow dust from the Gobi Desert. Dust from Africa reaches cities in the US and Europe, leading to illnesses and worsening environments. |
| 1. Which of the following is Not mentioned in the passage? |
[ ] |
| A. The bad influences of deserts. B. How serious desertification is. C. What makes land turn into desert. D. The ways to change deserts into farms. |
| 2. The purpose of writing the first paragraph is to _______. |
[ ] |
A. serve as a picture of a large desert |
| 3. We can know from the passage that _______. |
[ ] |
| A. deserts can be easily seen in both hot and dry places B. one third of the area in Africa has become desert C. the UN has made many films on desertification in 2006 D. dealing with desertification needs world-wide cooperation |