The primary difference between a fixed budget and a variable (flexible) budget is that a fixed budget: 

(a). Includes only fixed costs, while a variable budget includes only variable costs. 

(b). Is concerned with only further acquisitions of fixed costs, while a variable budget is concerned with expenses which vary with sales. 

(c). Cannot be changed after the period begins, while a variable budget can be changed after the period begins. 

(d). Is a plan for a single level of sales (or other measure of activity), while a variable budget consists of several plans, one for each of several levels of sales (or other measures of activity).