Functionality is a characteristic associated with evaluating the quality of software products throughout their life cycle, and is BEST described as the set of attributes that bear on the: 
A、existence of a set of functions and their specified properties. 
B、ability of the software to be transferred from one environment to another. 
C、capability of software to maintain its level of performance under stated conditions. 
D、relationship between the performance of the software and the amount of resources used.