Which of the following is not a rule for conducting orthogonal contrasts? a. The sum of weights for a comparison should not be zero. b. The group that is singled out in one comparison should be excluded from any subsequent contrasts. c. For a given contrast, the weights assigned to the group(s) in one chunk of variation should be equal to the number of groups in the opposite chunk of variation. d. If we give a group a weight of zero then this eliminates that group from all calculations.