The company claims that the mineral supplement can effectively prevent the common cold. A scientist disputes the claim, investigation.
Which change to the investigation would remove the source of this error?
OA Test a fourth group, in which participants do not receive the mineral supplement.
B. Test all three groups, with each participant receiving the mineral supplement for 12 months.
C. Test only one group, with each participant receiving 200 mg/day of the mineral supplement.
D. Test only two groups, one with each participant receiving 200 mg/day and one with 50 mg/day of the mineral suppler
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