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  3. Life near the shore everywhere is affected by the tides, which come and go twice each day in a cycle of about twelve-and-a-half

Life near the shore everywhere is affected by the tides, which come and go twice each day in a cycle of about twelve-and-a-half

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问题     Life near the shore everywhere is affected by the tides, which come and go twice each day in a cycle of about twelve-and-a-half hours—just different enough from the daily cycle of the sun so that there can be no regular relationship between the shore being alternately wet and dry and alternately light and dark. The extent of the tides varies greatly, from as little as one foot in inland seas like the Mediterranean, to fifty feet or so in the Bay of Fundy in Nova Scotia. In some parts of the world, one of the two daily tides rises higher and falls lower than the other; and tides at the time of new moon and full moon are generally greater than at other times. The extent of the intertidal zone thus varies from day to day as well as from place to place.
    The kinds of organisms living in the region between the tidal limits depend very much on whether the shore is rocky, sandy, or muddy. Rocky shores have the most obviously rich faunas (某地区所有的动物)，because of the firm anchorage for both animals and plants, and because of the small pools left by the retreating seas. Sandy shores, especially when exposed to surf (as they usually are), have the fewest kinds of animals.
The time span between tides______.
选项 A、is shorter in inland seas
B、varies as much as the extent of the tides
C、and the extent of tides depend upon each other
D、is a more constant phenomenon than the extent of the tides
答案D
解析 文中第一段说：“……潮汐按12小时半的循环规律一天来去两次，这样就与太阳的每日一循环形成了足够的时间差，所以海岸干与湿的交替与天空明与暗的转换也就不可能形成有规律的联系。潮汐规模的变化很大……”由此我们可知，两次潮汐的间隔期比潮汐规模更有恒值性。
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