You will listen to part of a lecture and answer a question about it. After you h

Now listen to part of a lecture in a history class.
    So designing tall buildings… first off, obviously, is that when architects design tall buildings they have to make sure the bottom of the building is strong enough to support all the weight at the top, but the way architects accomplished this in the past is different from the way they do it today, so let’s talk about how their approach has changed.
    In the past, when people built tall buildings, they generally used very heavy materials such as stone. But because they were using such heavy materials, the architects needed to design thick walls at the bottom, you know, at the base, to support all that weight at the top. Using thick walls at the bottom was a successful method for supporting the enormous weight of the building, but it had a drawback. The walls were so thick that they limited the amount of space inside the building. For example, think about a tall stone tower. Stones are very heavy of course, so there’s a lot of weight pushing down on the bottom of the tower, so the walls at the bottom have to be very thick and because the walls are so thick and take up so much space, the rooms inside the tower are small and cramped.
    But nowadays, tall buildings are built with stronger materials that are much lighter than the building materials in the past. Because of this, architects can design tall buildings with relatively thin walls and large spacious rooms. For example, think of a tall office building that’s made of steel. If a building this tall were made of stone, the walls would have to be very thick to support all that weight. But since steel is relatively much lighter than stone, the top of the building isn’t that heavy. Steel is also very strong; a relatively thin steel wall can support a lot of weight. So nowadays, the walls of this tall building don’t have to be nearly as thick as they had to be in the past.