moving up and down．This is why an object on top of a wave will bob up and down，but not move forward．

　　When a wave reaches the shore，however, the water does move forward．It surges(前进)onto the shore．This is because the "bottom" of the wave drags on the sand and the "top" continues on，crashing onto the shore．This crashing water is called the surf．Most waves are caused by wind blowing across the ocean's surface．The size of a wave depends on how fast and how far the wind blows over the surface and on the depth of the wave．Small winds can cause ripples，while strong winds create large hurricane waves．

　　Along most shorelines．water levels rise and fall twice a day．These changes，called tides．are caused by the gravitational pull of the Sun and the Moon．The ocean surface lifts，or bulges(凸出)，in two places：on the side of Earth that faces the Moon and on the side of Earth that faces away from the Moon．Both bulges cause a high tide on nearby shorelines．At the same time that the high tides occur，low tides occur between the two bulges．

　　Tides are also affected by the pull of the Sun，although the effect of the Sun is not as great as the effect of the Moon．When the Moon and Sun are lined up with Earth，the combined pull is the strongest，causing the highest high tides and the lowest low tides．These very high and very low tides are called spring tides．Weaker tides，called neap tides，happen when the Moon and the Sun are at right angles to Earth．High tides alternate with low tides．Along most shorelines，a high tide or low tide occurs about every six hours．

21．The word "alternate" in the last paragraph means ______．

A．appear suddenly B．bob up and down

C．change or become quickly D．happen or follow one after another

22. The water in the ocean is actually moving ______.

　A. left and right B. forward and backward C. here and there D. up and down

23. Most waves are caused by ______.

A. the sun B. the air C. the wind blows D. fish in the ocean