sea当“海水”的意思讲的时候,是不可数的。
举例,The sea is as blue as the sky.
sea当“海”（个数）讲的时候,是可数的。
举例,There are five seas in our province.
After ten days at sea, we had our first sight of land.
我们在海上航行十天之后，首次看见陆地。
The sea wall was not proof against the strength of the waves.
防波堤挡不住海浪的力量。
The smell of the sea carried her back to her childhood.
大海的气息勾起了她童年的回忆。
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