Science, as we think, was born when the Greek philosopher (  ) Thales ( about 640-546 B.C.) asked a difficult question: What makes up our universe? No one had a ready answer, so Thales went on studying the earth around him, the sky and the stars. He saw so much water on earth and so much water falling from the sky as rain that he decided water must be the basic substance (  ) of the universe. Other Greek thinkers became interested in this question. They suggested other answers. One said that because air lies around the earth, it must be air that makes up all things. Another said that fire, appearing in different forms, was the building block of the universe. The Greek philosophers were feeling their way towards the ideas on which chemistry is based. Centuries later, scientists proved that the universe is made up of certain basic substances. But the list is much more _ than the Greeks realized. We now know of 103 basic substances which we call " elements (  )". After reading the passage, we can be sure that  _  .
nothing ever changes in the universe Thales decided that the basic substance of the universe was air the universe is made up of four different substances the early Greek thinkers did much valuable work for the progress of science