State whether the following studies are experimental (E) or observational (O)
a. ___ A balanced study of polio rates between an equal number of boys and girls taken randomly from the census of 1920.
b. ___ A study of the effect of exercise on sleep, where subjects responded to the study solicitation and reported their degree of exercise and level of sleep.
c. ___ A sleep study where 6 subjects sleep 3 times in the sleep lab under one of 3 ambient sound conditions, western classical music beach surf, or silence, randomly assigned each visit.