Critical Reasoning (CR) | Five years ago, during the first North American outbreak

Five years ago, during the first North American outbreak of the cattle disease CXC, the death rate from the disease was 5 percent of all reported cases, whereas today the corresponding figure is over 18 percent. It is clear, therefore, that during these past 5 years, CXC has increased in virulence.

Which one of the following, if true, most substantially weakens the argument?

(A) Many recent cattle deaths that have actually been caused by CXC have been mistakenly attributed to another disease
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