The （1） 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 （2）.
（1）A. waterfall model
B. workflow reference model
C. evolutionary model
D. spiral model
（2）A. workflow engines
B. processes
C. workflow threads
D. tasks