B4

It's very difficult to believe the fact the first skyscrapers

 ONE

B5

were built in Chicago during the 1880s.

 BUILD

B6

They were the result of a need for more working and living space

 MUCH

B7

in some American big cities

 CITY

B8

where the cost of living was very high.

 BE

B9

Instead of using a lot of expensive space on the ground their builders used the free space of the sky.

 THEY

B10

The walls of the early skyscrapers often were made of stone - not for practical reasons,

 MAKE

B11

but to make the buildings look solid and strong.

 BUILDING

B12

So the most famous symbols of America appeared more than a century ago.

 APPEAR