请阅读Passage One，完成第21—25小题。 Passage One Coasts mark the area where dry land meets oceans or other large bodies of water．They are one of the most active environments found on Earth．The coasted landscape is ever changing． It reflects the conflicting processes erosion of and deposition．It is a landscape that is affected by the interac-tions of the Earth，the Moon，and the Sun． Coasts are among the most beautiful and inspiring landscapes on the planet，whether they are scenes of torrential storms or serene calm．Throughout recorded history，humans have sought to live on or near coasts． Although coasts account for only lo percent of Eaah’s land surface，they serve as home to tw0．thirds of the world’s human population． Coasts and shore．coastline and shoreline，are commonly used in place of each other．Techni- cally，they are different areas along a coastal hmdscape．The line that marks‘the boundary between water and land is the shoreline． It constantly fluctuates because of the regular action of waves and tides．The shore is that strip of ground borderi ng a body of water that is alternately covered or ex- posed by waves or tides：The boundaries of the shore are marked by the shoreline at its farthest sea-ward(10w tide)and farthest landward(high tide)． Tides is the Deriodic rising and falling of water in oceans and other large bodies of water in re- sponse to the gravitational attraction of the Moon and the Sun upon Each．Although the Sun is larger than the M00n．the Moon is closer to Earth and，therefore，its gravitational force is approximately2．2 times greater．The gravitational pull of the Moon creates two types of tides：high and low．A tidal bulge occuYs in the oceans on the side of Each nearest the Moon，at the same time，a second tidal bulge occurs on the opposite side of Earth．This second bugle forms because the force of the Moon’s gravity pulls the solid body of Eaah slightly away from the water on Eaah’s far side．These bugles are high tides．The areas between the tidal bugles experience low tide． As Earth rotates．the tides move over its surface．It takes approximately 24 hours and 50 mi-nutes for a given point on Earth to make a complete cycle relative to the Moon，and two lunar tidal cvcles occur during this time．Thus，the average time between high tides is 1 2 hours and 25 mi-nutes．This is a generalized explanation of titles．They do not move evenly and predictably over Earth’s surface．Variations in the depth of the oceans and the distribution of landmasses combine with other factors to produce highly complex tidal behavior． The coast and coasdine begin where the shore ends at its high tide mark(farthest landward)．The line between the coast and the shore at high tide is the coastline．The coast extends landward from the coastline to the first major change in terrain features，which may be miles inland．This could be a highland or a forest or some other type of terrain．Sometimes，the change between the coast and the adjacent terrain is not so distinct． One lunar tidal cycle lasts__________．