According to paragraph 1 what has happened in Maur

单项选择题

Questions 57 to 60 are based on the following passage: Only one generation ago, Mauritania’s capital city was many days’ walk from the Sahara. Today it is in the Sahara. The sand blows through the city streets and piles up against walls and fences. The desert stretches out as far as the eye can see. In some parts of the Amazon rain forest in Brazil, all the trees have been cut down. The earth lies bare and dry in the hot sun. Nothing grows there anymore. Over vast areas of every continent, the rainfall and vegetation necessary for life are disappearing. Already more than 40 percent of the earth’s land is desert or desert-like. About 628 million people---one out of seven--live in these dry regions. In the past, they have managed to survive, but with difficulty. Now, largely through problems caused by modem life, their existence is threatened by the slow, steady spread of the earth’s deserts. Many countries are experiencing similar problems. Poor land is farmed until it is worn out, and trees are cut for firewood, leaving the soil unprotected against wind and rain. In Peru, Chile, and Brazil, some areas that once were covered with forests now look like the moon. In India, some land has been so badly damaged by farming and tree cutting that mud now slides into the Indus and Ganges rivers. Cattle, sheep, and goats add to the problem by eating grass and other plants faster than they can grow back. In the United States, some highly populated areas (such as Los Angeles) are really deserts. Water must be piped in from hundreds of miles away--and this affects the water supply of other California communities. Scientists still do not understand all the complex problems of the desert, but there have been many ideas for saving the land. Saudi Arabia has planted 10 million trees to help keep the sand from taking over fertile areas. The Israelis are again using some of the water collection systems left by the ancient peoples in the Negev desert. They plan to water their orchards with the extra water. Some Sahel farmers still raise cattle on their poor farm land, but before the cattle are sold, they are taken to greener lands in the south to get fat.

According to paragraph 1 what has happened in Mauritania

A．The Sahara has spread until it has reached Mauntama capital.
B．Nothing has happened
C．It takes many days to walk through the Sahara.
D．Mauritania has moved its capital city into the Sahara.