碳酸二甲酯被称为绿色化学试剂，可以代替光气、硫酸二甲酯等剧毒原料，同时它还是良好的溶剂和燃油添加剂．碳酸二甲酯的化学式为C₃H₆O₃，试计算：

（1）碳酸二甲酯的相对分子质量为______；

（2）碳酸二甲酯中碳、氢、氧三种元素的质量比为（化成最简整数比）______；

（3）碳酸二甲酯中碳元素的质量分数为______．

It should go without saying that the focus of UML is modeling. However, what that means, exactly, can be an open-ended question. (1) is a means to capture ideas, relationships,decisions, and requirements in a well-defined notation that can be applied to many different domains. Modeling not only means different things to different people, but also it can use different pieces of UML depending on what you are trying to convey. In general, a UML model is made up of one or more (2) . A diagram graphically represents things, and the relationships between these things. These (3) can be representations of realworld objects,pure software constructs, or a description of the behavior of some other objects. It is common for an individual thing to show up on multiple diagrams; each diagram represents a particular interest, or view, of the thing being modeled. UML 2.0 divides diagrams into two categories: structural diagrams and behavioral diagrams. (4) are used to capture the physical organization of the things in your system, i.e., how one object relates to another. (5) focus on the behavior of elements in a system. For example, you can use behavioral diagrams to capture requirements, operations, and internal state changes for elements.

A．Activity diagrams

B．Use-case diagrams

C．Structural diagrams

D．Behavioral