Why Is Random Assignment Critical for Research Studies

Why is random assignment critical for research studies?

A) Because it ensures that on average the groups in an experiment will be similar with respect to all possible variables when the experiment begins
B) Be­cause it helps to establish that correlation is the same as causation and that these terms are interchangeable
C) Because it restricts the possible answers in a survey to some kind of a numeric rating scale
D) Because conditions in study groups cannot be controlled and cause-and-effect relationships between variables cannot be demonstrated