An experiment is conducted examining the effects of a new type of toothpaste on reducing cavities. Individuals in the treatment condition are given the new toothpaste; individuals in the control condition are given the standard toothpaste, but are told that they're receiving the new toothpaste. At the end of the experiment, the control group participants demonstrate a slightly higher decline in cavities. This is an example of a(n):____.
a. Wording bias.
b. Voluntary response bias.
c. Over-sampling effect.
d. Placebo effect.
d. Hawthorne effect.