洁净手术间的相对湿度应保持在：（）。

A.30～50%

B.30～40%

C.40～50%

D.40～60%

A work breakdown structure (WBS) numbering system should allow project staff to ______ .

A) identify the level at which individual WBS elements are found
B) identify configuration management milestones
C) estimate the costs of WBS elements
D) provide project justification