(71) analysis emphasizes the drawing of pictorial system models to document and validate both existing and/or proposed systems. Ultimately, the system models become the (72) for designing and constructing an improved system. (73) is such a technique. The emphasis in this technique is process-centered. Systems analysts draw a series of process models called (74) 、 (75) is another such technique that integrates data and process concerns into constructs called objects.

（74）应选（）。

A．PERT

B．DFD

C．ERD

D．UML

作业环境一氧化碳最高允许浓度为（）mg/m³或（）。