Assuming Sales Prices and Cost Behaviour Remain Unchanged, When Variable

Assuming sales prices and cost behaviour remain unchanged, when variable costing is used, when does net income change in response to changes in unit sales?

A) only when number of units sold exceeds number of units produced
B) only when number of units produced exceeds number of units sold
C) only when number of units sold exactly equals number of units produced
D) under all the above conditions