To measure the effectiveness of new pain reliever, a group of patients in pain clinic are randomly selected to receive a dose of the new medication or the placebo. In order to reduce the effect of any bias, neither the patients nor doctors know who gets what type of treatment. At the end of six weeks, all the patients were asked to rate their pain level. What kind of study is stated here?
a) Observational, case control
b) Cross sectional study
c) Double blind experiment
d) Sample survey