Estimating schedule activity costs involves developing an (1) of the cost of theresources needed to complete each schedule activity. Cost estimating includes identifying and considering various costing alternatives. for example, in most application areas, additionalwork 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 important information about project requirement that is considered during cost estimating.空白（2）处填（）

A. activity
B. work
C. currency
D. time