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根据短文内容，选择最佳答案，并将选定答案的字母标号填在题前括号内。

给出定义：若$m-\frac{1}{2}<x\le m+\frac{1}{2}$（其中m为整数），则m叫做离实数x最近的整数，记作{x}，即{x}=m在此基础上给出下列关于函数f（x）=|x﹣{x}|的四个命题：

①$f\left(-\frac{1}{2}\right)=\frac{1}{2}$；②f（3.4）=﹣0.4；

③$f\left(-\frac{1}{4}\right)=f\left(\frac{1}{4}\right)$；④y=f（x）的定义域为R，值域是$-\frac{1}{2},\frac{1}{2}$；

则其中真命题的序号是（　　）

塔里木盆地边缘的山麓地带和河流沿岸，分布着星星点点的____ ，它们的水源主要来自____和____。

昆虫是动物界中种类最多、数量最大的一个动物类群，它属于（    ）