题目

如图所示，一只茶杯静止在水平桌面上，则（ ）A. 茶杯受到的重力和桌面对茶杯的支持力是一对平衡力B. 茶杯受到的重力和茶杯对桌面的压力是一对平衡力C. 桌面受到的支持力和茶杯对桌面的压力是一对平衡力D. 桌面受到的重力和茶杯对桌面的压力是一对平衡力 答案：【答案】A【解析】二力平衡的条件是：二力大小相等、方向相反、作用在一条直线上、作用在同一个物体上。A. 茶杯受到的重力和桌面对茶杯的支持力，二力大小相等、方向相反、作用在一条直线上、都作用在茶杯上，是一对平衡力；B. 茶杯受到的重力和茶杯对桌面的压力，二力方向都向下，且没有作Have you ever heard a news reporter talk about DNA? Reporters talk about DNA found at the scene of a crime. They talk about police finding DNA “fingerprints.” Police sometimes use DNA as a clue to find out who committed the crime.DNA is a substance（物质） that makes up genes. Everything alive has genes. Plans have genes. Animals have genes. You have genes.Genes are the basic units of heredity（遗传）. Heredity means all the characteristics you inherit from your parents. You get your genes from your parents. You inherit half of your genes from your mother. You inherit half of your genes from your father.Genes are a kind of code. A tree’s genes tell what shape its leaves will be. A cat’s genes tell what color its fur will be. Your genes tell what color your eyes will be. Your genes tell what color your hair will be. Everything about you comes from the code in your genes.Genes line up on strands（链） called chromosomes（染色体） in cells. Everything alive is made up of cells. Chromosomes are in the center, or nucleus, of cells.Different parts of you are made of different kinds of cells. Your muscles are made of muscle celIs. Your skin is made of skin cells. The code in your genes tells your body to make different kinds of cells. The genes in each cell tell the cell how to work. They tell the cell when to make new copies of itself.An Austrian monk named Gregor Mendel first saw inherited patterns in pea plants. He experimented with pea plants in the 1860s. One of the things, or traits（特质）， Mendel studied was what makes some pea plants tall and some short. He said that the traits must come from units of heredity passed from the parent plants. These units were later called genes.In the mid-1900s, scientists discovered that genes are made of DNA. In the 1970s, scientists learned how to change DNA with genetic engineering. Scientists also learned that problems with certain genes cause diseases. Muscular dystrophy, cystic fibrosis, and hemophilia are some genetic diseases-diseases caused by problems in genes. Today, scientists are looking for ways to cure genetic diseases by changing genes through a process called gene therapy.小题1: What is DNA?A．DNA is a kind of gene.B．DNA is a substance that makes up genes.C．DNA is the basic unit of heredity.D．DNA is a measure to protect crime.小题2: Which of the following about genes is correct according to the passage?A．Plants, animals and human beings have the same genes.B．Half people inherit all genes from their mother, others from their father.C．Genes decide the trees shapes, the cats’ fur color and our eyes’ color as well.D．Genes will give you a code when you need them.小题3:Where are genes?A．Genes lining up on strands called chromosomes are in the center of cells.B．Genes hide in everything alive in your body.C．Genes can be nowhere but in your mind, controlling all your actions.D．Genes travel in your body and help cope skin, muscle, and eyes.小题4:An Austrian monk named Gregor Mendel         .A．first saw inherited patterns in peopleB．was interested in why plants were differentC．first called the units of heredity from parents genesD．was the first who discovered genes小题5: We can conclude that         .A．scientists were less intelligent than monks in 1900sB．some genes are bad and can cause diseasesC．we don’t need to worry about genetic diseases any longerD．the discovery of genes may be of great help in our daily life