It is estimated that over 99 percent of all species that ever existed have become

　　extinct. What causes extinction? When a species is no longer adapted to a changed

　　environment, it may perish. The exact causes of a species’ death vary from situation

　　Line to situation. Rapid ecological change may render an environment hostile to a species.

　　For example, temperatures may change and a species may not be able to adapt. Food

　　Resources may be affected by environmental changes, which will then cause problems

　　For a species requiring these resources. Other species may become better adapted to an

　　Environment, resulting in competition and, ultimately, in the death of a species.

　　The fossil record reveals that extinction has occurred throughout the history of Earth.

　　(10) Recent analyses have also revealed that on some occasions many species became extinct

　　at the same time—a mass extinction. One of the best-known examples of mass extinction

　　occurred 65 million years ago with the demise of dinosaurs and many other forms of life.

　　Perhaps the largest mass extinction was the one that occurred 225 million years ago,

　　When approximately 95 percent of all species died, Mass extinctions can be caused by

　　(15) a relatively rapid change in the environment and can be worsened by the close

　　interrelationship of many species. If, for example, something were to happen to destroy

　　much of the plankton in the oceans, then the oxygen content of Earth would drop,

　　affection even organisms not living in the oceans. Such a change would probably lead to a mass extinction.

　　One interesting, and controversial, finding is that extinctions during the past 250

　　Million years have tended to be more intense every 26 million years. This periodic

　　extinction might be due to intersection of the Earth’s orbit with a cloud of comets, but

　　this theory is purely speculative. Some researchers have also speculated tat extinction

　　may often be random. That is, certain species may be eliminated and others may survive

　　(25) for no particular reason. A species’ survival may have nothing to do with its ability or

　　inability to adapt. If so, some of evolutionary history may reflect a sequence of essentially

　　random events.

　　20. The word “it” in line 3 refers to

　　(A) environment

　　(B) species

　　(C) extinction

　　(D) 99 percent

　　21. The word “ultimately” in line 8 is closest in meaning to

　　(A) exceptionally

　　(B) dramatically

　　(C) eventually

　　(D) unfortunately

　　22. What does the author say in paragraph 1 regarding most species in Earth’s history

　　(A) They have remained basically unchanged from their original forms.

　　(B) They have been able to adapt to ecological changes.

　　(C) They have caused rapid change in the environment.

　　(D) They are no longer in existence.