请问为什么是D
13. 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 that mimics the symptoms of CXC.
(B) During the first North American outbreak of the disease, many of the deaths reported to have been caused by CXC were actually due to other causes.
(C) An inoculation program against CXC was recently begun after controlled studies snowed inoculation to be 70 percent effective in preventing serious cases of the illness.
(D) Since the first outbreak, farmers have learned to treat mild cases of CXC and no longer report them to veterinarians or authorities.
(E) Cattle that have contracted and survived CXC rarely contract the disease a second time.
我从新解释一下。原文的推理是：cxc报告的死亡率从5%增大到18%〉cxc毒性加强
死亡率是=死亡的数量/报告样本的总数
D说的是一次爆发以后农场主 have learned to treat mild cases of CXC and no longer report them to veterinarians or authorities，这样实际报告的样本总数减少，导致死亡率被夸大。从而削弱了提干的结论。
我觉得Since the first outbreak可能不包括第一次。
第一次发病死亡率＝death1：CXC1=5%
第二次发病死亡率＝death2：CXC2=18%上升
mild cases of CXC 肯定指没有造成死亡的。所以death2比较真实，从死亡率2上升不能推出CXC2数据是否上升（却能推出下降）。削弱。
