Which of the following distinguishes an observational study from a randomized experiment? In an observational study volunteers are always used, whereas in a randomized experiment a random sample is always taken from the population.

In an observational study volunteers are always used, whereas in a randomized experiment a random sample is always taken from the population.
A

In an observational study a random sample is always taken from the population, whereas in a randomized experiment volunteers are always used.

In an observational study a random sample is always taken from the population, whereas in a randomized experiment volunteers are always used.
B

In an observational study treatments are not randomly assigned, whereas in a randomized experiment treatments are randomly assigned.

In an observational study treatments are not randomly assigned, whereas in a randomized experiment treatments are randomly assigned.
C

In an observational study a control group is never used, whereas in a randomized experiment a control group is always used.

In an observational study a control group is never used, whereas in a randomized experiment a control group is always used.
D

An observational study can be double-blind, whereas a randomized experiment can only be single-blind because the experimenter determines who is randomly assigned to each treatment.