Thanks for the question! This is one of the all-time great #% LSAT questions, and it's a perfect learning tool to better understand this concept. Let's start with an analysis of the stimulus:
Premise: 5 years ago the death rate from CXC was 5% of all reported cases.
Premise: Now the death rate is 18%.
Conclusion: CXC has gotten more harmful/deadlier.
At a glance, this is a pretty simple stimulus: the death rate went up, so the disease is getting worse. However, arguments that are based solely on numbers or solely on percentages can be dangerous. Why? Because they only reflect part of the story. There are various explanations of the basic concepts in the lesson and homework, but as a quick example consider the following, which I often picture as happening in the old Wild West:
Business owner: Last year, our horse-trading business held about 10% of the market in town. This year, we hold over 50%.
Reporter: Wow, business must be going great!
If you think about this, what are the problems with the conclusion that business is going great? In other words, what info is missing from that example? First, what's happening to the population of the town (which corresponds to the missing number info). If, for example, the town had a sudden population decrease, that percentage growth looks far less impressive. Let's say the town became a near-ghost town, and lost most of it's population. so maybe last year they had 500 clients out of a population of 5,000, and this year they have 50 clients out of 100. If so, that would call into question the idea that business is going great just because they have 50% of the market.
Or, instead of a population drops, what about if the overall the general demand dropped? When the first cars came out, the use of horses began dropping steadily. So, the town might be the same size, but maybe everyone switched over the driving a car and they don't need horses anymore. That also would lead to a much reduced market size, and would make that 50% figure very deceptive.
So, turning back to the CXC problem, what's missing or notable here? First, we know nothing of the actual number of cows that is being reviewed. All the argument says is "reported cases." Second, "death rate" is just one element of a disease, and perhaps the illness is much less severe for the cows that did not die. Prior to reading the answers we can't be sure which direction the test makers will go, but it's scenarios like this that should flash through your mind when you see that Weaken question stem.
Let's look at each answer choice:
Answer choice (A): This strengthens the argument. If this is true, then it suggests the 18% number is too low, and should actually be higher.
Answer choice (B): This also strengthens the argument. If this is true, then it suggests the 5% number was too high, and should actually be lower.
Answer choice (C): If anything, this would strengthen the argument, but really this answer addresses an issue that's more or less "after the fact" of the data in the stimulus. We don't know how fast or widely distributed the inoculations are, but if they've had any effect yet, it would be to make that 18% lower than what it would have been otherwise. That, then, would strengthen the author's position.
Answer choice (D): This is the correct answer. If this is the case, it affects the "report cases" aspect of this problem. Basically, the scenario works like this: 5 years ago, when CXC was new, farmers reported every case because they didn't really understand CXC. So, all cases—including mild cases and severe cases—were reported. 5 years later, farmers know how the disease works, and when they see a minor case, they don't worry too much and they don't report them. They are now only reporting the severe cases. Well, the severe cases are the ones that are most likely to lead to death, and so naturally you'd expect to see a higher percentage of those cases dying. Numerically, maybe 5 years ago you had 100,000 cases reported, and 5% died. Now, you have only 10,000 reported, but 18% died. So, the higher percentage doesn't mean the disease is getting worse. You can't tell without knowing the actual numbers, and consequently, the conclusion is undermined.
Answer choice (E): This isn't much help because it's about the second time rate of contraction. The author would still argue that the percentages show it's getting worse.
Please let me know if that helps. Thanks!