听力原文：If a city ever needed tall buildings in a hurry, it was Chicago in the 1880s. Loc

听力原文： If a city ever needed tall buildings in a hurry, it was Chicago in the 1880s. Located in the center of America's farmland, with rail links to every coast, it was the natural hub of commerce for the entire continent, and one of the fastest growing cities in the country. The great Chicago Fire of 1871 made the need for tall buildings urgent even before the 1880s. The fire raged for two days and wiped out one-third of all the buildings in the city. The fire was so destructive that when Architect William LeBaron Jenney began work on the Home Insurance Building twelve years later, rebuilding was still underway. According to legend, Jenney was working on his design for the Home Insurance Building one afternoon in1883 when a bird began making so much noise that he couldn't concentrate. He got so angry that he grabbed the heaviest book he could find and pounded furiously on the bird's steel-wire cage to shut it up. The cage could have broken after such abuse, Jenney thought afterwards, but it didn't. It didn't even dent. If steel cages were so strong, he realized, why not make buildings out of steel？ Why not build the Home Insurance Building with steel？ But in the early 1880s, buildings were still rarely taller than six or seven stories, and it wasn't just because people hated slow elevators or climbing stairs, Bricks, the standard construction materials of the time, were too heavy to build much higher than that. To support a tall structure, the lower walls would have to be so thick that there would be little floor left. Besides, why use so many bricks to add just one floor when the same number of bricks could be used to construct an entire building somewhere else？ Steel, on the other hand, is so much lighter and can carry so much more weight than brick that you can build more than 100 floors before you run into the same type of problem.

(30)

A．They used bricks and they are too heavy.

B．They used bricks that were too expensive.

C．Steel was in short supply.

D．The benefits of steel were unknown to them.