Two of the most damaging gasses currently released into the atmosphere are: a. chlorofluorocarbons (CFOs) and fossil fuels
b. fossil fuels and methane
c. the ozone layer and nitrous oxide
d. CFOs and CO2
e. CO2 and methane

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.