Which of the following is not found in an observational study? A-controlled variable B-manipulated variable C-None of these are found in an observational study. D-control group

C-None of these are found in an observational study

Explanation:

(A.) controlled variables are a part of what's found in a controlled experiment. (B.) manipulated variables are a part of what's found in a controlled experiment. (C.) Answer choice A. or Controlled variables, Answer choice B. or Manipulated variables, and Answer choice D. or Control groups are a part of what's found in a controlled experiment. (D.) Control groups are a part of what's found in a controlled experiment. Therefor this only leaves one answer and that answer is Like I said C-None of these are found in an observational study.

b.

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explanation:

your answer is "the 1950's"