Configuration management is the process of managing change in hardware，software，firmware，documentation，measurements，etc．As change requires an initial state and next state，the marking of significant states within a series of several changes becomes important．The identification of significant states within the revision history of a configuration item is the central purpose of （） identification.

A．baseline

B．value

C．cost

D．control

已知Rt△ABC外接圆半径为R，内切圆半径为r，则△ABC的周长为（    ）。