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13. Our great-grandparents used "mustard plasters" as a cure for chest colds. Which would be good scientific evidence for or against the effectiveness of this treatment?
a) Data from controlled experiments on the use of mustard plasters for cold sufferers.
b) A prescription for mustard plasters in an old medical book.
c) A chemical study of mustard to see what is in it.
d) A letter from a doctor describing what a mustard plaster does.
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