Net operating income under absorption costing may differ from net operating income determined under marginal costing. How is this difference calculated?

1. change in the quantity of units in inventory times the fixed manufacturing overhead rate per unit.
2. number of units produced during the period times the fixed manufacturing overhead rate per unit.
3. change in the quantity of units in inventory times the variable manufacturing cost per unit.
4. number of units produced during the period times the variable manufacturing cost per unit.