Which of the following is undesirable for psychological research and theories?

At middle age, there is an apparent decrease in height caused by

A. intervertebral disk degeneration B. decreased thickness of joint cartilage in the lower extremities C. abnormal spinal curvature D. all selections are correct

In experimental studies,

A) the investigator cannot make cause-and-effect inferences. B) investigators must take special precautions to control for participants' characteristics that could reduce the accuracy of their findings. C) the researcher directly controls or manipulates changes in the dependent variable. D) researchers gather information on participants without altering their experiences.