Passage 2 Questions 6 to 10 are based are on the following passage. 　It is difficult to form a correct idea of a desert without having seen one. It is a vast plain of sands and stones, with mountains here and there of various sizes and heights, without roads or shelters. They sometimes have springs of water. The most remarkable of deserts is the Sahara. This is a vast plain, but little elevated above the level of the ocean, and covered with sand and gravel（砂砾）, with a mixture of sea shells. 　Amid the desert, there are springs of water, which burst forth and create verdant（翠绿的） spots, called oases. There are thirty-two of these which contain fountains, and date and palm trees; twenty of them are inhabited. They serve as stopping places for the caravans, and often contain villages. Were it not for these, no human being could cross this waste of burning sand. So violent, sometimes, is the burning wind that the scorching （灼热的） heat dries up the water of these springs, and then frequently, the most disastrous consequences follow. 　In 1805, a caravan consisting of 2,000 persons and 1,800 camels, not finding water at the usual resting place, died of thirst, both men and animals. Storms of wind are more terrible in this desert than on the ocean. Vast surges and clouds of red sand are raised and rolled forward, burying everything in there and it is said that whole tribes have thus been swallowed up. 　The situation of such is dreadful, and admits of' no resource. Many die and become victims of the most horrible thirst. It is then that the value of a cup of water is really felt. To be thirsty in a desert, without water, exposed to the burning sun, without shelter is the most terrible situation a human being can be placed in. 　If unfortunately, any one falls sick on the road , he or she must either endure the fatigue of traveling on a camel, which is troublesome even to healthy people, or he or she must be left behind on the sand, without any assistance, and remain so until a slow death comes to relieve him or her.The first paragraph tells us that________.