单项选择题 某学校学生、教师和课程实体对应的关系模式如下： 学生(学号，姓名，性别，年龄，家庭住址，电话) 课程(课程号，课程名) 教师(职工号，姓名，年龄，家庭住址，电话) 如果一个学生可以选修多门课程，一门课程可以有多个学生选修；一个教师只能讲授一门课程，但一门课程可以有多个教师讲授。，由于学生和课程之间是一个(51)的联系，所以(52)。又由于教师和课程之间是一个(53)的联系，所以(54)。

单项选择题 A．They should help maintain the equipment.B．They should keep a watchful eye on their children.C．They should stop their children from climbing ladders.D．They should teach their children how to use the equipment.

单项选择题 听力原文： The United States has the largest economy in the world. The size of an economy is usually described in terms of the Gross Domestic Product. The Gross Domestic Product, or G.D.P., is the value of all goods and services produced in a country in a year. In 2004, the United States had a G.D.P. estimated at close to 12 million dollars. What percentage of that was agriculture? The government says just 9/10 of one percent. Farm workers make up an even smaller percentage of American labor: 7/10 of one percent. In 2004, almost 20 percent came from industry—and almost 80 percent came from services. The number of farms continues to decrease in America, whereas farm earnings have risen to record levels in recent years. Agricultural productivity continues to increase because of new technology and methods. But the Agriculture Department estimates that 9 percent of farm income last year came from government payments. That number is expected to decrease in the future. Exports have provided American farmers with an average of about 25 percent of their money for the last 8 fifteen years. Canada and Mexico are two of the three biggest markets for American farmers. In fact, in 2002, Canada replaced Japan for the first time as the top buyer of American agricultural exports. The Department of Agriculture says exports to the European Union are slowing. But exports to other countries within the Americas and to Asia are growing.(33)A．American economy.B．American agriculture.C．American industry.D．American GDP.