For many years, scientists couldn't figure out how atoms and molecules on the Earth combined to make living things. Plants, fish, dinosaurs, and people are made of atoms and molecules, but they are put together in a more complicated way than the molecules in the primitive ocean. What's more, living things have energy and can reproduce, while the chemicals on the Earth 4 billion years ago were lifeless.
　　After years of study, scientists figured out that living things, including human bodies, are basically made of amino acids and nucleotide bases. These are molecules with millions of hydrogen, carbon, nitrogen, and oxygen atoms. How could such complicated molecules have been formed in the primitive soup? Scientists were stumped.
　　Then, in 1953, two scientists named Harold Urey and Stanley L. Miller did a very simple experiment to find out what had happened on the Primitive Earth. They set up some tubes and bottles in a closed loop, and put in some of the same gases that were present in the atmosphere 4 billion years ago: water vapor, ammonia, carbon dioxide, methane, and hydrogen.
　　Then they shot an electric spark through the gases to simulate bolts of lightning on the ancient Earth, circulated the gases through some water, sent them back for more sparks, and so on. After seven days, the water that the gases had been bubbling through had turned brown. Some new chemicals were dissolved in it. When Miller and Urey analyzed the liquid, they found that it contained amino acids-the very kind of molecules found in all living things.
46. When did scientists come to realize how the atoms and molecules on the Earth combined to make living thing?
　　A. 4 billion years ago.     B.1953.     C. After seven days.     D. Many years later.
47. Scientists figured out that human bodies are basically made of .
　　A. amino acids                         
B. molecules
C. hydrogen, carbon, nitrogen and oxygen atoms
D. water vapor, ammonia, carbon dioxide, methane and hydrogen
48. Harold Urey and Stanley L.Miller did their experiment in order to .　
A. find out what had happened on the Earth 4 billion years ago
B. simulate bolts of lightning on the ancient Earth
C. dissolve some new chemicals
D. analyze a liquid
49. At the end of the last paragraph, the underlined word "it" refers to.
A. a closed loop         B. an electric spark         C. water         D. the liquid
50. According to the writer, living things on the Earth include .
　 A. atoms and molecules                      B. chemicals　
C. plants, fish, dinosaurs and human beings     D. the primitive soup