In the given passage about learning outcomes and private schools, choose the most appropriate conjunction to complete the clause blank it became difficult to empirically prove.

Introduction / Context:
This question is based on a passage discussing learning outcomes and comparisons between private and government schools. You are asked to choose the correct conjunction to fill the blank in the clause blank it became difficult to empirically prove that children learn better in private schools. Such questions test your understanding of how conjunctions link clauses logically in a sentence and maintain the correct flow in a longer passage.

Given Data / Assumptions:

The fragment under focus is blank it became difficult to empirically prove that children learn better in private schools.
The options are So, Therefore, When, and Then.
The surrounding context talks about how a certain argument changed once it became hard to prove a claim about private schools.

Concept / Approach:
We need a word that introduces a time based clause and fits before it became difficult. The word when is a subordinating conjunction used to connect a time clause to a main clause, meaning at the time that. So and therefore are used to show results or conclusions, usually at the beginning of an independent clause, not before a subject like it in this structure. Then can be an adverb of time or result, but would sound broken when placed directly before it became difficult. The passage is describing what happened at the time it became difficult to prove a point, so when is the natural choice.

Step-by-Step Solution:
Step 1: Read the full sentence in the passage: blank it became difficult to empirically prove that children learn better in private schools, the attack invented a new weapon. Step 2: Recognise that the sentence describes a change that occurred at a particular time in the argument. Step 3: Evaluate when, which gives the sense of at the time that it became difficult, and see that it easily introduces a subordinate time clause. Step 4: Consider so and therefore, which normally start a result clause and are usually followed by a comma or directly by a clause, not structured as so it became difficult in a subordinate way. Step 5: Note that then functions as an adverb rather than a conjunction here and would not comfortably fit at the beginning of a subordinate clause, so select when as the best option.

Verification / Alternative check:
Insert each option into the sentence and read it as part of the passage. When it became difficult to empirically prove that children learn better in private schools, the attack invented a new weapon reads smoothly and clearly marks the time relationship between the two actions. So it became difficult and therefore it became difficult both sound more like the start of independent result clauses and do not fit naturally before the rest of the sentence. Then it became difficult lacks the subordinating role required here, leaving the main clause without a clear connector. This confirms that when is the only choice that aligns with both grammar and the logical flow of the passage.

Why Other Options Are Wrong:
Option A So is mainly used to show consequence or result and would normally be placed before a main clause, not as a subordinator before it became difficult. Option B Therefore is a conjunctive adverb used to introduce a conclusion and is not suitable to form a dependent time clause. Option D Then works as an adverb of time or sequence, but it does not create a subordinate clause linked to the main statement in a natural way.

Common Pitfalls:
Students sometimes overuse connectors like so and therefore whenever they see cause and effect relationships without checking whether the grammar of the sentence allows them. Another pitfall is ignoring the larger passage structure and treating the sentence fragment in isolation. A good strategy is always to reconstruct the full sentence mentally and see which option creates a natural and grammatically complete relationship between the clauses. Remember that when commonly introduces time clauses and fits smoothly before expressions like it became difficult.

Final Answer:
When