A Randomized Experiment Was Done by Randomly Assigning Each Participant

A randomized experiment was done by randomly assigning each participant either to walk for half an hour three times a week or to sit quietly reading a book for half an hour three times a week. At the end of a year the change in participants' blood pressure over the year was measured, and the change was compared for the two groups.
-This is a randomized experiment rather than an observational study because

A) blood pressure was measured at the beginning and end of the study.
B) the two groups were compared at the end of the study.
C) the participants were randomly assigned to either walk or read, rather than choosing their own activity.
D) a random sample of participants was used.

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