Within-Groups Degrees of Freedom in a Two-Way ANOVA Is Calculated

Within-groups degrees of freedom in a two-way ANOVA is calculated by:

A) multiplying together the degrees of freedom associated with each of the main effects.
B) adding together the degrees of freedom associated with each of the main effects.
C) subtracting the degrees of freedom for the first main effect from that of the second main effect.
D) subtracting a single participant from each cell of the study and then adding the results for all the cells.