Mill's Method of Residues Is Used When Part of an Effect

Mill's method of residues is used when part of an effect (E) can already be explained by certain causal factors (such as a and b) .It provides evidence for an additional causal factor, c, by:

A) directly observing c causing E
B) showing that E can still occur in the absence of a or b
C) showing that E can still occur when both a and b are absent
D) showing that c is more important than a or b
E) showing that a and b only explain part of E