Often organisms seem similar in their outward appearances. for example, a porpoise and a shark seem closely related, but they are not. such similarity is the result of a. genetic equilibrium. b. convergent evolution. c. adaptive radiation. d. artificial selection.

In an experiment examining the effects tai chi on arthritis pain, callahan (2010) selected a large sample of individuals with doctor-diagnosed arthritis. half of the participants immediately began a tai chi course and the other half (the control group) waited 8 weeks before beginning. at the end of 8 weeks, the individuals who had experienced tai chi had less arthritis pain that those who had not participated in the course.