It should go without saying that the focus of UML is modeling. However, what that means, exactly, can be an open-ended question. __(71)__ 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 __(72)__. A diagram graphically represents things, and the relationships between these things. These __(73)__ can be representations of real-world 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. __(74)__ are used to capture the physical organization of the things in your system, i.e., how one object relates to another. __(75)__ 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．views

B．diagrams

C．user views

D．structure pictures

流体阻力产生的根本原因是由于流体与壁面之间的摩擦引起的。