Câu hỏi:

Read the following passage and mark the letter (A, B, C or D) on your answer sheet to indicate the correct answer to each for the question.

 Tides are the periodic rise and fall of the Earth’s waters that are caused by the Moon’s and Sun’s forces of gravity acting on the Earth .It is important to distinguish natural tidal phenomena from huge tsunamis , with the latter being caused by earthquakes and undersea volcanic eruptions.

 The Moon is a main factor controlling ordinary tides. At the location on the Earth closest to the Moon, it exerts a powerful gravitational pull on the water. The resulting rise in the water produces higher tides. The water on the side of the Earth farthest away from the Moon also gets pulled by this lunar gravity, but not as strongly. The Earth itself has its own gravitational force that is constantly pulling waters downward, which is why the oceans do not simply bulge out toward the Moon. Ordinary tides usually feature high and low waters alternating in relation to the Earth’s rotation. Most shores around the world have high waters and two low waters for each day, which last about 24 hours and 50 minutes. The difference in height between the high water and low water is called the range of tide, and it can be quite dramatic in narrower bays .Canada’s bays of Fundy , for example , commonly experiences the world’s most extreme tidal ranges , with daily differences of the 16 meters.

 Two other types of tides are influenced by the Sun, which is much farther away from the Earth and exerts less than half of the Moon’s gravitational force. When the Sun, the Moon and the Earth are directly in line, the solar and lunar gravitational forces add up to produce higher spring tides. The range of spring tides is intensified, with higher water marks and lower low water marks. However, when the Moon is in the first or third quarter, it is at a 90–degree angle with the Sun in relation to the Earth .The opposing solar and lunar forces partially cancel each other out, and the result is a lower tide. This is called a neap tide, which comes twice a month and has lower high water marks and higher low water marks. The range of neap tides is minimum.

 Some tides do not occur over water at all. The solid body of the Earth has slight elasticity, so lunar and solar gravity cause it to stretch very subtly. These changes in the Earth’s shape, although imperceptible to humans, are known as Earth tides. Another tidal phenomenon, atmospheric tides, is caused by the Sun’s heating of the Earth’s atmosphere. Like ordinary tides, they usually occur over 12–hour periods. 

Why does the author mention tsunamis in the passage?