For a Third Variable to Be Plausible as the Explanation

Multiple Choice

For a third variable to be plausible as the explanation in an established association, which of the following must also be true?

A) The third variable must be related to both of the measured variables in the original association.
B) The third variable must be measured on the same scale as the original measured variables.
C) The third variable must be a categorical variable.
D) The third variable must have a positive relationship with the two measured variables in the original association.