What is the primary difference between a fixed budget and a flexible budget? a. A fixed budget includes only fixed costs, while a flexible budget includes only variable costs. b. A fixed budget is concerned only with future acquisitions of fixed assets, while a flexible budget is concerned with expenses, which vary with sales. c. A fixed budget cannot be changed after the period begins, while a flexible budget can be changed after the period begins. d. A fixed budget is a plan for a single level of sales (or other measure of activity), while a flexible budget consists of several plans, one for each of several levels of sales (or other measure of activity).