The stimulus presents an argument. The principal concludes that teaching has improved on the basis of the fact that, after she complained that student failures were caused by bad teaching, failing grades disappeared.
Well, who is responsible for assigning failing grades? The teachers! So maybe the teachers just stopped assigning failing grades - this doesn't mean teaching has improved, right?
Answer choice (D) has the same flaw. A manager claims that workers who file complaints don't have enough to do. Shortly thereafter, no more complaints were filed. The workers themselves, however, are the ones who were responsible for the complaints. Just because they stopped complaining does not mean they are now "productively filling their time."
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