酒后驾驶是一种违法行为。交通警察常用装有硫酸酸化的重铬酸钾（K₂Cr₂O₇）的仪器检测司机是否酒后驾驶，因为乙醇分子可以使红色的重铬酸钾变为绿色硫酸铬。重铬酸钾中铬元素的化合价为_______。

Traditional structured analysis techniques focus upon the flow of (71) within a system. Object - oriented analysis emphasizes the building of real - world models. It examines requirements from the perspective of the classes and objects found in the vocabulary of the (72) domain.

Traditional system design method emphasizes the proper and effective structure of a complex system. Object -oriented design method encompasses the process of object -oriented decomposition and a (73) for depicting both logical and physical as well as static and dynamic models of the system under design.

Object - oriented programming is a method of implementation in which programs are organized as cooperative collections of objects, each of which represents an (74) of some class, and whose classes are all members of a hierarchy of classes united via (75) relationships.

72（）

A．problem

B．solution

C．data

D．program