3. According to Weber’s law, the amount of increase in the intensity of a stimulus needed to produce a just noticeable difference (jnd) is a function of the original intensity of the stimulus. To test the hypothesis that relative to starting with unsweet tea, starting with an already sweet tea causes one to have to add larger amounts of sugar to produce a jnd, you should use which of the following types of research designs?

a. An experiment

b. A correlational design

c. A case study

d. All of the above designs would work equally well.