题目

已知数列{an}为等差数列，若 且它的前n项和 有最大值，那么取最小正数时n的值是（    ） A.22                    B.23                    C.24                    D.25 答案：ＢAlthough men and women have been growing crops for thousands of years, they have not always had enough to eat. Even today, many people in the world go hungry, and even die of hunger. If we are to grow enough for everybody, we have to grow more crops and better crops.Plants need the right food in order to grow. In particular, they need three chemical substances (物质) in the soil: nitrogen (氮), phosphorus (磷) and potassium (钾). They also need smaller amounts of other chemical substances like calcium. If there is too little of any of these substances in the soil, plants can’t grow well.When a plant grows, it takes up these basic substances from the soil and builds them into itself. If we remove the plant from the soil and use it for food, we are taking some of these substances away, and this means that future plants will find less of them in the soil. In time, the soil would become too poor for plants to grow in it at all.To prevent this  we must put basic substances back. One way of doing this is putting manure (肥料) on the soil. This contains the remains of plants that have been eaten, and, if we add it to the soil, we are returning some of the basic substances. In Europe and in Japan, manure from animals has for long been used in this way. Unfortunately much of the animal waste in India is used as fuel because there is not enough wood to burn. This means there is less manure for the crops.Even when we use manure, we can not put back into the soil more of the basic substances than have been taken out. Often the soil does not have enough of them in the first place, and so we must provide extra supplies. We can use chemical fertilizers (化肥). Experiments in India have shown that, if we use a fertilizer containing a mixture of nitrogen and phosphorus, we get half as much rice again from the crops on any one piece of land.72. According to the passage, how many methods are introduced to improve the soil?A. One.         B. Two.          C. Three.          D. Four.73. The underlined word “this” (in Paragraph 4) refers to the fact that ____________.  A. the soil would become too poor for plants to grow in it at all  B. we are taking some substances away from the soil  C. the plant takes up these essential substances from the soil  D. even today many people in the world go hungry74. To make sure that the soil has enough basic substances, people can _____________.    A. remove plants from the soil              B. provide the soil with extra supplies   C. plant as many plants as possible          D. stop planting in the soil75. The passage is mainly about _____________.    A. why more food is needed B. how to grow better crops   C. the fact that people have not always enough to eat   D. the essential substances that plants need