(paragraph 1) Several theories have been proposed about why the dinosaurs disappeared from the face of the earth. In recent years, one popular theory proposes that climatic changes caused the dinosaurs to become extinct. This climatic change theory says that millions of years ago the climate of the world gradually became colder. As the earth slowly become colder fewer plants were able to grow. The cold weather finally resulted in a severe shortage of food for dinosaurs. As you probably know, most of dinosaurs were vegetarians, and they depended on plants for their food supply. In summary, then, the disappearance of the dinosaurs was cased directly by a shortage of plants to eat, and indirectly by a change in climate.
(paragraph 2) Today there is new evidence for the theory, that the dinosaurs did not disappear gradually, but , they disappeared quickly and suddenly. This theory is known as asteroid theory. It states that a huge asteroid, or perhaps a comet, hit the earth about 65 million years ago. When this comet or asteroid hit the earth, it caused a huge dust cloud. The huge dust cloud covered the whole earth and blocked out the sun for months. Since there is no sun for many, many months, most of the plants on earth died. The dinosaurs’ food supply was destroyed in a period of months. .
(paragraph 3) While this asteroid theory is not new, what is new is the evidence for the theory. Until recently there was no evidence that an asteroid or a comet had hit the earth 65 million years ago. What happened recently was that scientists found large amount of a rare earth element called iridium all over the world.
(paragraph 4) This iridium was found in layers of the earth that are 65 years old. The iridium was found in the same layers where the bones of the last dinosaurs were found.
(paragraph 5) The comet or asteroid theory explains two things: (1) It explains the larger amounts of the rare element iridium found in the 65-million-year-old layers of earth, and (2) it explains why the dinosaurs disappeared from the earth. Today scientists continue to debate two theories: the climatic change theory and the asteroid theory. In the future evidence may be found that supports a totally new theory of why the terrible lizards died out.
(Quetion) The main information of the passage is about .......