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.

多项选择题 铸石管主要特点是（）。