How has the amount (level) of carbon dioxide in the atmosphere been changing over the past 1000 years?
question options:
carbon dioxide levels have remained the same.
carbon dioxide levels have decreased.
carbon dioxide levels cannot be measured.
carbon dioxide levels have increased.

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.