题目内容：
The ____ is a general description of the architecture of a workflow management system used by the WFMC, in which the main components and the associated interfaces are summarizeD．The workflow enactment service is the heart of a workflow system which consists of several ____. In a workflow system, a process is specified using the process definition tool. Usually, processes are defined in terms of a ____, which consists of ____ indicated using a circle, and __请作答该选项__ shown as a rectangle. These two components can be linked by means of a directed arC．
A.trasitions
B.transformations
C.transmitters
D.changes
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