An important new industry, oil refining(炼油), grew after the Civil War．Crude oil,or petroleum--a dark, thick ooze from the earth--had been known for hundreds of years.But little use had ever been made of it．In the 1850s Samuel M．Kier, a manufacturer in western Pennsylvania, began collecting the oil from local seepages( 油苗)and refining it into kerosene(煤油)．Refining, like smelting(锻造), is a process of removing impurities from a raw material. Kerosene was used to light lamps．It was a cheap substitute for whale oil, which was becoming harder to get．Soon there was a large demand for kerosene．People began to search for new supplies of petroleum, The first oil well was drilled by E．L． Drake, a retired railroad conductor．In 1859 he began drilling in Titusville, Pennsylvania．The whole venture seemed so impractical and foolish that onlookers called it 'Drake's Folly'．But when he had drilled down about 70 feet (21 meters), Drake struck oilhis well began to yield 20 barrels of crude oil a day. News of Drake's success brought oil prospectors(探寻者)to the scene, By the early 1860 s these wildcatters(盲目采油者)were drilling for 'black gold' all over western Pennsylvania．The boom rivald the California gold rush of 1848 in its excitement and Wild West atmosphere,And it brought far more wealth to the prospectors than any gold rush. Crude oil could be refined into many products.For some years kerosene continued to be the principal one．It was sold in grocery stores and door to door．In the 1880s and 1890 s refiners learned how to make other petroleum(石油) products such as waxes(石蜡)and lubricating oils (润滑油)．Petroleum was not then used to make gasoline or heating oil. 第 31 题 What is the main idea of the passage?