Under Which Circumstance(s) Should You Avoid Using Analysis of Covariance

Multiple Choice

Under which circumstance(s) should you avoid using analysis of covariance?

A) If you need to control for all extraneous variables on which the groups may differ.
B) If you have identified variables that have not been controlled for.
C) If you have controlled for all possible variables.
D) If you need to reduce unexplained variance.