Forty people with sore backs are randomly divided into two groups. One group is treated weekly with acupuncture and an exercise regimen. The other is treated weekly with exactly the same exercise regimen, but no acupuncture. After one year, both groups are questioned about their sore backs.
Was this an observational study or an experiment? Why?
Choose the correct description of the study.

a) The study is an observational study because the researchers only observe values of the response and explanatory variables.
b) The study is an observational study because the researchers cannot control the response variable after 1 year.
c) The study is an experiment because the researchers assign subjects to certain experimental conditions and observe the outcomes on the response variable.
d) The study is an experiment because the researchers cannot control the response variable after 1 year.