The______of an expression is not a thing, so it is difficult to

The______of an expression is not a thing, so it is difficult to say what sort ofidentity it is. For example, and, if, yes don’t refer to anything in the world, but all have meanings.
【正确答案】：sense
【题目解析】：从题干看，这里谈的是意义三个层面的概念中的一个---sense。