摘要： from other continents for millions of years, Australia has many plants and animals not found in any other country in the world. A. Being separated B. Having separated C. Having been separated D. To be separated 第二大题 完形填空 Pearls are valuable white gems which come from the ocean. Actually they are 16 by oysters, small shell fish that live on the 17 of the ocean. 18 oysters will make pearls; not all oysters produce them. Oysters produce pearls only when they are 19 by sand. If a 20 of sand enters the oyster's 21 , the oyster becomes hurt because the 22 grain of sand irritates the smooth, soft skin of the oyster. The oyster tries to 23 itself by producing a white 24 that looks like milk. The oyster covers the 25 with a milky fluid. The fluid protects the oyster. Later the white liquid become 26 . This hardened liquid forms a shell, or bead, around the sand. At this time a pearl is beginning to 27 . The white pearl grows 28 inside the oyster's shell. 29 , it takes about six or seven years for the oyster to produce a pearl. Not all oysters produce pearls 30 most oysters sometimes take sand into their 31 . Only sand which the oyster cannot get rid of will 32 it. In other words, if an oyster "swallows" some sand, it will manage to "spit it out". If the oyster cannot get rid of the 33 , then it will produce the white fluid to 34 itself. Actually, only about one in a thousand oysters will produce a pearl; that is, 35 than one percent.

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