If a researcher uses a careful matching procedure in correlational research, why should we

still not draw conclusions about cause-and-effect relations between variables?

a. The measures may be unreliable.
b. A third variable influencing the relationship may have been missed.
c. The sample may not be a representative one.
d. Participants' expectations may interfere with their behavior.
e. The Hawthorne effect may occur.

b. A third variable influencing the relationship may have been missed.