Passage Summary: A principal claims that bad teaching causes students to fail. When students stop failing, the principal assumes it is because the teachers are now doing a better job.

Conclusion: The quality of teaching at the school has improved.

Reasoning: The principal believes that bad teaching is the cause of student failure, and failing grades have recently disappeared from the school.

Analysis: The argument suffers from a classic logical error: it assumes that because an effect (failing grades) has vanished, the specific cause identified by the principal (bad teaching) must have been removed. This ignores the possibility that other factors changed, such as the school lowering its grading standards or students simply studying harder. To match this flaw, look for an answer choice that observes the disappearance of a problem and concludes that one specific potential cause must have been corrected, while ignoring alternative explanations. It is a classic case of confusing a sufficient condition for a necessary one.