Julia is working on creating the project budget. She is considering the activity costs, work package costs and control account costs to create the project budget. To create a budget, activity costs are rolled up into work package costs. Then work package costs are rolled up into control account costs, and then final project costs. Julia is rolling up costs from the work package level to the control account level which is called?

A. Cost estimation.

B. Cost aggregation.

C. Forecasting.

D. Cost performance index.