A、Life on other planets. B、Vegetation on Earth. C、Mammals in deeper seas. D、Minerals in the earth crust. A

When I tell people what my particular branch of science is, I often get funny looks. In a way, I understand because astrobiology is the study of life on other planets. Well, obviously, life has not been discovered on other planets, which would appear to make astrobiology a science without a subject! However, everything we know about life on our own planet suggests we have to try to understand if there are any universal requirements for life to evolve, as well as the processes involved in evolution. Consequently, scientists are deeply interested in the beginnings of life on Earth. Once we know more about what happened on home ground, as it were, we will be in a better position to understand any life forms we may one day find on other planets.
    When most people think of extra-terrestrial life, they conjure up images of so-called "higher" life forms: they imagine humanoid creatures or bizarre and probably dangerous animals of some kind. But if we consider the whole history of life on Earth, a very different picture emerges. For billions of years the only forms of life on the planet were organisms consisting of single cells. It was only about 550 million years ago that the seas suddenly became filled with a whole array of multicellular life.
    So how do humans fit into this time frame? Well, human-like creatures first appear in the fossil record about five or so million years ago: in geological terms, this is just a blink of an eye compared to the long history of life on Earth. And Homo sapiens, our own species, has only been around for about 130,000 years. The point is if we do find life on other planets, it will almost certainly be relatively simple, of the sort that populated the Earth for most of its existence so far.
    And of course, we must be prepared for these life forms to look very different from life on Earth. We must not forget that many modern life forms came about as a result of chance, their fate shaped by floods, continental drift and comet or meteor strikes. It is interesting to reflect that if a giant asteroid had not hit the Earth and wiped out the dinosaurs, they might still be ruling the planet and we might never have evolved.
Question No. 16 Which of the following does astrobiology study?
Question No. 17 When telling people what branch of science he studies, why does the speaker often get funny looks?
Question No. 18 With what life forms did the seas suddenly become filled some 550 million years ago?
Question No. 19 For how long has Homo sapiens, our own species, has been existing on Earth?
Question No. 20 Many modern life forms came about as a result of a series of chance factors. Which of the following is NOT one of these factors mentioned in the talk?