Estimating schedule activity costs involves developing an（1 ）of the costs of the resources needed to complete each schedule activity. Cost estimating includes identifying and considering various costing alternatives. For example, in most application areas, additional work during a design phase is widely held to have the potential for reducing the cost of the execution phase and product operations. The cost estimating process considers whether the expected savings can offset the cost of the additional design work. Cost estimates are generally expressed in units of     （2 ）to facilitate comparisons both within and across projects. The（3 ）describes the business need, justification, requirements, and current boundaries for the project. It provides important information about project requirements that is considered during cost estimating.
（1）A. accuracy        B. approximation       C. specification       D. summary
（2）A. activity         B. work                C. currency           D. time
（3）A. project scope statement                  
B. statement of work
C. project management plan                 
D. project policy