Activity based costing

hello I have a doubt relating to variable o/h costs. First of all, this is what I have understood about variable production and variable O/H costs: 1. Variable production costs a. Short-term variable production costs – cost driver based on the volume of activity b. Long term variable production costs – ABC based cost drivers 2. variable O/H costs a. Short-term variable O/H costs – ABC-based cost drivers b. Long term variable O/H costs - cost driver based on the volume of activity Sir, Now here are my questions to you: Q1 why don't we use cost drivers based on the volume of activity for Long term variable production costs? Q2 why are long-term variable O/H costs are not allocated using ABC-based cost drivers? Thankyou!