Five years ago, during the first North American outbreak of the cattle

OFFICIAL EXPLANATION

(A) No. This substantially strengthens the argument. If many recent deaths due to CXC have been mistakenly attributed to another disease, then the 18 percent death rate would be even higher.

(B) No. This substantially strengthens the argument. If many of the deaths reported to have been caused by CXC were actually due to other causes, then the 5 percent initial death rate would be lower. Hence, the increase in virulence would be even greater.

(C) No. This choice is irrelevant. The effectiveness of an inoculation program has no bearing on the virulence of a disease. Inoculations are used to prevent infection by a disease. However, once infected, the disease’s virulence measures the severity of the sickness. The fact that a disease can often be prevented does not affect its virulence if caught.

(D) Yes. If the mild cases are no longer reported, then the serious cases would be disproportionately represented. For example, suppose of 100 cattle with the disease, 73 have a mild case, 27 have a serious case and 5 will die of the disease. Then the death rate would be 5 percent. Now, if the 77 cattle with mild cases are not considered, then the reported death rate would be approximately 18 percent: 5/27 ≈ 18%.

(E) No. The passage is about the virulence of the disease, not about an animal’s immunity to a second infection.