Which Statement BEST Reflects the Third-Variable Problem

Which statement BEST reflects the third-variable problem?

A) A correlation among three variables increases the difficulty of determining which variable is most influential.
B) When a third variable is measured in a correlational study, the strength of the correlation between variable #1 and variable #2 typically diminishes.
C) A correlation between two variables may occur because some third variable is responsible for the relationship between the two variables.
D) Although adding a third variable to a correlational study increases predictability, it makes it more difficult to draw cause-effect conclusions.