The essential difference between an experiment and an observational study is that a. observational studies always involve large numbers of subjects, but experiments never dob. in an experiment, information is gathered only on animals or things, but in an observational study, only information about people is gatheredc. an experiment imposes treatment on the subjects, but an experiment does not
d. an observational study imposes treatments on the subjects, but an observational study does not

An experiment imposes treatment on the subjects, but an observational study does not

Explanation:

Observational studies are research studies in which an effect, process behaviour etc. are observed, without any change. For example, observation of the  possible effect of a treatment on subjects.

On the other hand, experimental studies are studies in which the treatment is included. Experimental studies are randomized, meaning that subjects that are in experiment are randomly put to a treated group or a control group.