Professor west has 30 participants drink either 0 mg, 2 mg or 4 mg of caffeine mixed in juice before completing two tasks; a response time task and a memory task. in this experiment, the independent variable is:

d. i assume the underlined word is "role." reformat if it isn't.

explanation:

role is closest to part because when replacing words, "playing a vital decision," "playing a vital activity," and "playing a vital dollar" don't make logical sense. the only meaningful choice is "playing a vital part."

this fits one of merriam-webster's definitions of role: "a function or part performed especially in a particular operation or process."

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