A、To show how simple the mechanical aids for diving can be. B、To provide an excuse for his changeable character. C、To explore th

When my interest shifted from space to the sea, I never expected it would cause such confusion among my friends, yet I can understand their feelings.
    As I have been writing and talking about space flight for the best part of 20 years, a sudden switch of interest to the depth of the sea doer seem peculiar. To explain, I’d like to share my reasons behind this unusual change of mind. The first excuse I give is an economic one. Underwater exploration is so much cheaper than space flight. The first round-trip ticket to the moon is going to cost at least 10 billion dollars if you include research and development. By the end of this century, the cost will be down to a few million. On the other hand, the diving suit and a set of basic tools needed for skin-diving can be bought for 20 dollars. My second argument is more philosophical. The ocean, surprisingly enough, has many things in common with space. In their different ways, both sea and space are equally hostile. If we wish to survive in either for any length of time, we need to have mechanical aids. The diving suit helped the design of the space suit. The feelings and the emotions of a man beneath the sea will be much like those of a man beyond the atmosphere.
Questions 17 to 20 are based on the passage you have just heard.
17. How did the speaker’s friends respond to his change of interest?
18. What is one of the reasons for the speaker to switch his interest to underwater exploration?
19. In what way does the speaker think diving is similar to space travel?
20. What is the speaker’s purpose in giving this talk?