Which of the Following Is NOT True of Third Variables

Multiple Choice

Which of the following is NOT true of third variables and mediating variables?

A) Third variables are external to the causal variable, but mediating variables are internal to the causal variable.
B) Third variables are considered nuisances, but mediating variables are not.
C) Third variables can be detected using multiple regression techniques, but mediating variables cannot.
D) Third variables are not usually of central interest to researchers, but mediating variables are.