It's very difficult to believe the fact that the __ (one) skyscrapers __ (build) in Chicago during the 1880s. They were

It\'s very difficult to believe the fact that some of the skyscrapers were built in Chicago during the 1880s. They were the result of a need for more working and living space in some American big cities where the cost of living was very high. Instead of using a lot of expensive space on the ground their builders used the free space of the sky. The walls of the early skyscrapers were often made of stone - not for practical reasons, but to make the buildings look solid and strong. So the most famous symbols of America appeared more than a century ago.