Which two statements about forces are true? A. Every force has an equal but opposite force.
B. Every force requires matter to touch.
C. Forces are always balanced.
D. Forces organize and affect how matter moves.

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.