(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.

A．PERT
B．DFD
C．ERD
D．UML

下列说法正确的是：    

A．P和S属于第三周期元素，P原子半径比S小

B．Na和Rb属于第ⅠA族元素，Rb失电子能力比Na强

C．C和Si属于第ⅣA族元素，SiH₄比CH₄稳定

D．Cl和Br属于第ⅦA族元素，HClO₄酸性比HBrO₄弱