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  3. Tides are created mainly by the pull of the moon on the earth. The moon’s pull causes water in the oceans to be a little deeper

Tides are created mainly by the pull of the moon on the earth. The moon’s pull causes water in the oceans to be a little deeper

admin2010-12-06  47
问题    Tides are created mainly by the pull of the moon on the earth. The moon’s pull causes water in the oceans to be a little deeper at a point closest to the moon and also at a point farthest from the moon, on the opposite side of the earth. These two tidal "waves" follow the apparent movement of the moon around the earth strike nearly every coastline at intervals of about twelve hours and twenty-five minutes. After reaching a high point, the water level goes down gradually for a little more than six hours and then begins to rise toward a new high point. Hence, most coastlines have two tides a day, and the tides occur fifty minutes later each day. Differences in the coastline and in channels in the ocean bottom may change the time that the tidal wave reaches different points along the same coastline. The difference in water level between high and low tide varies from day to day according to the relative positions of the sun and the moon because the sun also exerts a pull on the earth, although it is only about half as strong as the pull of the moon. When the sun and the moon are pulling along the same line, the tides rise higher, and when they pull at right angles to one another, the tide is lower. The formation of the coastline and variations in the weather are additional factors which can affect the height of tides. Some sections of the coast are shaped in such a way as to cause much higher tides than are experienced in other areas. A strong wind blowing toward the shore may also cause tides to be higher.  
Which of the following may be concluded from the information presented in the passage?
选项 A、Some coastlines do not have two tides each day.
B、Tides usually rise to the same level day after day.
C、Tides are not affected by the shape of a coastline.
D、The sun has as much effect on tides as does the moon.
答案A
解析 根据短文提供的信息可以断言某些海岸线并不是每天出现两次潮汐。文中说，大多数海岸线每天出现两次潮汐(Most coastlines have two tides a day.)。这就意味着有些海岸线并非每天出现两次潮汐。
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