The (29) is a general description of the a

问题：

The (29) 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 (30) 。

29（）

A. waterfall model     

B. workflow reference model

C. evolutionary model

D. spiral model