【单选题】

Chicago began life in 1779 (1) a small trading post on theChicago River.A、farsighted black freedman, Jean du Sable, did a flourishing fur (2) with the NativeAmericans. When his trading post became a fort and then a city, it was (3) Chicago. This is the NativeAmerican word for the wild onions found in the areA、

In 1820Chicago (4) ten or twelve houses and a store or two. Now it is the third largest city in the (5) , and still it continues to grow. Thousands of new buildings are (6) every year. They are built to accommodate new businesses and residences. More than seven (7) people now live in and around the city.
Astride the crossroads of the nation,Chicago is the largest railroad (8) in the worlD、No other city in the land is a larger trucking center. The city, (9) on lake Michigan, is the largest inland port in the worlD、O’HareAirport is the world’s busiest commercial airport. Overall,Chicago is the leading (10) center in the United States.
Its location in the heart of NorthAmerica’s farmland (11) Chicago the world’s largest grain market. It also plays (12) each year to more than a thousand conventions. In 1860Abraham Lincoln was nominated for President at the Republican convention (13) here. That was to establish a pattern for both theDemocratic and Republican parties since that time. Half of all major (14) conventions have taken place inChicago.
ButChicago did not achieve success without problems.At one time pollution from theChicago River (15) the city’s water supply from Lake Michigan. (16) pure drinking water, the engineers reversed the course of the river (17) it flowed backwards, away from the lake! This kept the water supply (18) .Even the famous fire of 1871 could not snuff out the spirits of the vital young giant. The entire central city was (19) , but citizens built anew.And they erected the first towering structure of steel and concrete. In doing so, they invented the (20) Today, as an example,Chicago’s impressive skyline includes the world’s tallest building. The 1454 foot Sears Tower.
A、EnsuredB、To ensure C、EnsuringD、To be sure