_____ describes functionality of the software to be constructed, the core of which is to describe the interaction between the actor and the system, but not the internal activity of the system.
_____描述了要构建的软件系统的功能，其核心是描述参与者和系统之间的交互，而不是系统的内部活动。

A．Collaboration diagram
B．State diagram
C．Use case diagram
D．Class diagram