When should a company use an activity-based flexible budget with multiple cost drivers instead of a simple

Question:

When should a company use an activity-based flexible budget with multiple cost drivers instead of a simple flexible budget with one cost driver?
a. When a significant portion of costs vary with only one cost driver
b. When a significant portion of costs vary with the number of units of output
c. When a significant portion of costs vary with the number of units of sales
d. When a significant portion of costs vary with cost drivers other than units of output