As most people know that there is a twice-daily occurrence with two high tidesand two low tides taking place within a single 24-hour period. The role played by themoon (primarily) and the sun (secondarily) was not widely recognized until Sir IsaacNewtown proposed his theory on the gravitational pull of celestial bodies.      Although factors such as the shape of the coastline, meteorological conditions,and the rotation of the Earth all act upon the oceans’ waters and affect the ultimateflow of the tides when they reach land, by far the most influential element is themoon. The gravitational attraction between the moon and the Earth’s waters is quitestrong, and the tides respond to the moon’s every movement. It acts like an invisiblehand that sets the waters into motion. Essentially, wherever the moon is in its orbitaround the Earth, seawater tends to “pile up”, like iron filings under a strong magnet.The pile of water remains in place directly under the moon as the Earth rotatesthrough its daily cycle. Of course, as the Earth spins, the relatively stationary pile ofwater under the moon eventually comes into contact with the continents. This is whatproduced a high tide.        The heights of the tides vary through the cycle of the lunar month. During a Newmoon, for example, the sun and the moon are arrayed in a line on the same side ofthe Earth, the gravitational force of the two is added together and the highest hightides of the month occur, called spring tides. When the sun, the moon, and the Earthare arranged in a triangular configuration, the pull of the two celestial bodies issomehow offset, causing the most moderate tides of the month, called neap tides.        Some have proposed building power generating plants to capture the energy ofthe oceans in the same way that the hydroelectric plants have been used to tap riverpower.
【題組】41. What is the main topic of the passage?
(A)The influential force of tides
(B)The significance of Newton’s theory
(C)The formation of tides
(D)The gravitational pull of the moon.