Determine whether the follow descriptions correspond to an observational study (OS), a controlled experiment (CE), or a double blind experiment (DBE). Write the abbreviations above as your answers. (without the parenthesis)

(a) A doctor gives half of his/her patients in a study a treatment and the other half a placebo, but the patients do not know which they received.
answer:

(b) A new study examines the effects of regular exercise on child obesity by taking a survey of 500 children on their weight and exercise habits.
answer:

(c) A nurse gives a group of patients either the drug being studied or a placebo; however neither the nurse nor the patient knows which they are receiving. Only the person who prepared the doses knows.
answer: