阅读下面短文，掌握其大意，然后从1－20各题所给的四个选项(A、B、C和D)中，选出最佳选项。

　　Pearls(珍珠)are valuable white gems(宝物)which come from the ocean．Actually they are 　　1　　 by oysters, small shell fish that live on the 　　2　　 of the ocean．

　　　　3　　 oysters(牡蛎，蚝)will make pearls; not all oysters produce them．Oysters produce pearls only when they are 　　4　　 by sand．If a 　　5　　 of sand enters the oyster's 　　6　　, the oyster becomes hurt because the 　　7　　 grain of sand irritates(使……不适，刺激)the smooth, soft skin of the oyster．The oyster tries to 　　8　　 itself by producing a white 　　9　　 that looks like milk．The oyster covers the 　　10　　 with a milky fluid．The fluid protects the oyster．Later the white liquid become 　　11　　．This hardened liquid forms a shell, or bead, around the sand．At this time a pearl is beginning to 　　12　　．

　　The white pearl grows 　　13　　 inside the oyster's shell．　　14　　, it takes about six or seven years for the oyster to produce a pearl．Not all oysters produce pearls 　　15　　 most oysters sometimes take sand into their 　　16　　．Only sand which the oyster cannot get rid of will 　　17　　 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 　　18　　, then it will produce the white fluid to 　　19　　 itself．Actually, only about one in a thousand oysters will produce a pearl; that is, 　　20　　 than one percent．

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