A、They could get protection from wild beasts. B、They wanted to move around easily. C、Natural shelters were very uncomfortable. D

Good morning, everyone. Now, today,(16)I am going to talk to you about the history of building and architecture. No story is more interesting or impressive than the story of man’s progress through the ages, and in particular the activities of human beings in the art of building. Let’s have a look at this in detail.
    In very early times, around 50,000 years ago, primitive humans lived in trees and caves, where they found protection from wild beasts and shelter from bad weather.(17)However, these natural shelters were pretty uncomfortable and so humans began to think of ways to construct more permanent dwellings, such as tents and huts.
    From these humble beginnings a great variety of architectural style gradually developed, and we see how humans began to master constructional difficulties and at the same time to achieve aesthetic desires. In other words,(18)we see how they began to create buildings that were not only functional but beautiful as well.
    Generally,(19)architecture is concerned with the enclosing of space. Another way of saying this is that architecture is about creating a safe, healthy and pleasant space— for the residents living and working there.
    There are three basic principles of architecture and I’d like to run over these now. The first is the principle of function; that is, the purpose of the building in question. The second is the principle of construction; how is the building to be built or constructed? And the third, after construction, is artistic expression.
16. What’s this passage mainly talking about?
17. Why did humans begin to think of ways to construct tents and huts?
18. What kind of building did man want to build after years of developing?
19. What is architecture concerned with according to the passage?