A city implemented a mandatory helmet law for all cyclists. A year later, the number of serious head injuries from cycling accidents reported by local hospitals had decreased by 20%. The city council therefore concluded that the helmet law was successful in reducing serious head injuries among cyclists. The city council's conclusion depends on which of the following assumptions?
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A
The number of cyclists and the overall frequency of cycling accidents did not significantly decrease in the year after the law was implemented.
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B
All cyclists in the city complied with the new mandatory helmet law.
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C
The 20% decrease in reported injuries is the most significant public health improvement the city has seen in the last year.
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D
Local hospitals did not change their criteria for what constitutes a 'serious head injury' during the period in question.