Let me give you some clues. It's a word of just three letters. You can't see it. You can't taste it. You can't hear or feel it. Nor can you smell it. And yet without it, we would not last very long. Got it? Yes, it's the stuff we all breathe in and out and it's called ?air?. Small it may be but it crops up in all sorts of places. 最後一句句法看起来好怪，请版上高手解答一下~ --

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