题目

 营业员昨天卖出7件衬衫和4条裤子，共560元，今天又卖出9件衬衫和6条裤子，共680元．若设每件衬衫的售价为x元，每条裤子的售价为y元，则可列方程组为___________． 答案：【解析】根据昨天和今天的销售情况，可得两个二元一次方程，他们组合起来就是所求方程组．We each have a unique genetic(基因的, 遗传的)make-up. Every cell of our body has the same set of about 100, 000 separate genes made of DNA. These are the instructions for producing a person. Genes decide everything from the colour of our skin to the way our brain works. We have one of several billion combinations of DNA which come from the random mixing of our parents’ genes. Except for identical twins(同卵双胞胎), no one has the same combination as another person. We are unique! We are unique in another way, too: in the way we are raised and all the experiences we have from before birth to adult life. These experiences influence us, our behaviour and attitudes, and the choices we make.But are genes or life experiences more important in shaping our appearance and personality? Scientists are studying twins to find out. One set of twins occurs every 70 births—some are identical and others are non-identical twins. Identical twins are special because they share exactly the same genes and often the same environment. Non-identical twins are more like ordinary brothers and sisters.Some identical twins have been adopted and brought up in different homes. With identical genes but a different home environment, scientists can study twins to see how much a particular feature depends on the genes we inherit(继承). For example, we know that eye problems, like short-sightedness, are mostly genetic. But resistance to pain is largely dependent on experiences. Genes also influence our eating habits. Identical twins brought up apart often like to eat at the same time of day and feel full after eating the same amount. Non-identical twins in similar circumstances have more varied eating habits. Identical twins are also more likely to follow the same patterns for marriage and divorce than non-identical twins.Scientists are trying to identify the different genes that influence our behavior. Some people are thrill-seekers and get into risk-taking and adventurous activities. They take up extreme sports like bungee jumping and possibly take drugs. Scientists have discovered a gene which affects this.We could ask, “Are our lives determined by our genes or our upbringing? ”Scientists are learning more all the time, but it is certainly true that both are important in making us who we are.1.What is the main idea of the first paragraph?A. Both our genes and our experiences make us who we are.B. How we turn out depends on our parents’ genes.C. Everyone has a physical double somewhere in the world.D. We cannot easily change our physical appearance.2.Why are scientists studying twins?A. To find out how many twins are born every year.B. To discover what shapes us as individuals.C. To compare differences between twins.D. To study brother-and-sister relationships.3. According to the passage, .A. one in 70 twins are genetically identicalB. non-identical twins are usually not of the same sexC. twins separated at birth behave exactly the sameD. identical twins are genetically the same4.What does the underlined word “this” in Paragraph 4 refer to?A. Our love of sports. B. Our attitude to risk-taking.C. Our skill at bungee jumping. D. Our ability to take drugs.