题目

The ocean is always moving：waves break on the surface, sea levels rise and fall with the tides, and currents flow below the surface. Although it looks as if the water in a wave is moving across the surface of the ocean, it is not. The water is actually 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 watera：b=2：3，那么，b：a=3：23：2．