美国每年需要大量进口的是[ ]

A．大豆和玉米    

B．棉花和小麦

C．热带作物    

D．小汽车和电子产品

The (71) 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 (72) . In a workflow system, a process is specified using the process definition tool. Usually, processes are defined in terms of a (73) , which consists of (74) indicated using a circle, and (75) shown as a rectangle. These two components can be linked by means of a directed arc.

（72）处填（）。

A．tasks

B．processes

C．workflow threads

D．workflow engines