Which of the following would be an example of a valid covariate in an ANCOVA analysis?

Which of the following would be an example of a valid covariate in an ANCOVA analysis?
a. IQ scores on students measured before implementing different reading comprehension programs.
b. Anxiety scores on subjects after implementing drug treatment interventions for
depression.
c. Racial bias of subjects prior to implementing a blood pressure reduction study.
d. Motivation level of subjects after implementing a smoking cessation intervention.

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