In the given passage about learning outcomes and private schools, choose the most appropriate verb form to complete the clause the attack blank a new weapon.

Introduction / Context:
This question continues the passage about arguments regarding private schools and learning outcomes. The sentence fragment to be completed is the attack blank a new weapon: per unit cost of learning outcomes. The passage narrates a sequence of events that has already occurred, so the correct verb form must fit that narrative past context. Such questions test both tense consistency and your ability to read a line as part of a larger logical argument.

Given Data / Assumptions:

The full sentence is similar to When it became difficult to empirically prove that children learn better in private schools, the attack blank a new weapon: per unit cost of learning outcomes.
The options are invents, had invented, is inventing, and invented.

The passage is written in past tense, describing how a new line of attack appeared once a certain claim could no longer be easily proved.

Concept / Approach:
The phrase the attack invented a new weapon is best expressed in simple past because it describes a completed action in the same time frame as became difficult. Using invents would move the sentence into simple present, breaking tense consistency. Had invented would indicate that the invention occurred even earlier than the difficulty in proving the claim, which is not intended. Is inventing would be present continuous and suggests an action in progress, again inconsistent with the narrative past tone. Therefore, invented is the verb form that properly matches both the temporal sequence and the style of a retrospective discussion.

Step-by-Step Solution:
Step 1: Note that became in When it became difficult is in simple past, so the surrounding context is past narration. Step 2: Recognise that the action of adopting a new argument or weapon follows this time frame and should also be described in simple past. Step 3: Evaluate invented as the simple past form of invent, which fits naturally: the attack invented a new weapon. Step 4: Check had invented and see that it would unnecessarily push the event further back in time than became, which the passage does not indicate. Step 5: Reject invents and is inventing as present tense forms that conflict with the past narrative, and choose invented as correct.

Verification / Alternative check:
Read the sentence with each option. With invented, we get When it became difficult to empirically prove that children learn better in private schools, the attack invented a new weapon: per unit cost of learning outcomes, which reads smoothly as historical narration of how the argument changed. With invents, the sentence awkwardly shifts into present tense and feels inconsistent with became. With had invented, the sentence suggests that the new weapon existed before the difficulty arose, which does not match the logic of cause and effect. With is inventing, the action seems ongoing now, which is clearly wrong in a reflective passage. Thus, invented is the only option that aligns with the intended meaning and grammar.

Why Other Options Are Wrong:
Option A invents is simple present and would require a different structure and context, such as a general truth, which is not the case here. Option B had invented is past perfect and misrepresents the sequence of events by placing invention before the difficulty, contrary to the passage logic. Option C is inventing is present continuous and is incompatible with the past tense narration of became difficult.

Common Pitfalls:
Learners sometimes overuse past perfect had plus past participle whenever they see more than one past event, even when simple past is sufficient. Another common error is not looking at the previous verb for tense guidance and choosing a form that sounds impressive but does not match the narrative. A helpful habit is always to identify the main timeline of the passage first and then select verb forms that keep the narration consistent unless a clear earlier or later time is explicitly indicated.

Final Answer:
invented