In the following passage about using a kerosene pump stove, some of the words have been left out. Read the passage carefully and select the correct answer for the given blank from the four alternatives. The description ends with the line was achieved. _________ you wanted the flame to be bigger, you had to hit the piston again. Pump stoves are still in vogue. In this question you must choose the correct conjunction to complete the condition was achieved. _________ you wanted the flame to be bigger.

Introduction / Context:
The last missing word in this kerosene pump stove passage is a conjunction that introduces a condition. The sentence reads in full as was achieved. _________ you wanted the flame to be bigger, you had to hit the piston again. The examinee must understand how the sentence links the condition wanting a bigger flame with the action of hitting the piston again. This is a classic test of basic conjunction use within a process description.

Given Data / Assumptions:

• The stove user has already adjusted the valve and achieved a desired flame size.
• Sometimes the user may want a bigger flame again.
• The incomplete sentence is was achieved. _________ you wanted the flame to be bigger, you had to hit the piston again.
• Options are If, Nor, So, and Thus.
• The sense required is conditional: in the case that you wanted the flame to be bigger, this is what you had to do.

Concept / Approach:
In English, the conjunction if is used to introduce a condition, especially in sentences of the type If you want X, you must do Y. The other words given here have different functions: nor is used with negatives, so usually introduces a result, and thus often signals a conclusion. Only if can introduce the conditional clause that fits the logic of the sentence. Therefore the correct completion is If you wanted the flame to be bigger, you had to hit the piston again.

Step-by-Step Solution:
Step 1: Recognise the structure of the sentence. The first part states that the desired flame size was achieved. The second part tells us what to do in a special situation. Step 2: Identify that the second part describes a condition: the special situation is that the user wants the flame to be bigger again. Step 3: Recall that conditional clauses in English typically begin with if, when, unless, or similar conjunctions. Step 4: Insert each option in the blank and check the meaning: If you wanted the flame to be bigger, Nor you wanted the flame to be bigger, So you wanted the flame to be bigger, Thus you wanted the flame to be bigger. Step 5: Note that only the first version with If is logical and grammatical, because it clearly introduces a condition that triggers the action of hitting the piston.

Verification / Alternative check:
A quick way to verify the answer is to rephrase the sentence as a standard conditional: If you wanted the flame to be bigger, you had to hit the piston again. This matches many other sentences you already know, such as If you wanted more light, you had to turn the knob, or If you wanted better pressure, you had to pump more. Nor, so, and thus cannot be used in this pattern, which confirms that if is the correct choice.

Why Other Options Are Wrong:
Nor: Nor is used in negative pairings, such as neither the stove nor the lamp worked. It cannot begin a positive conditional clause about wanting a bigger flame. So: So normally introduces a result or consequence, as in It was dark, so he lit the stove. Here we are not stating a result but introducing a hypothetical condition. Thus: Thus is a formal synonym for therefore and is used to state conclusions. The phrase Thus you wanted the flame to be bigger is awkward and does not link logically with had to hit the piston again.

Common Pitfalls:
Sometimes students see words like so and thus and think they always sound smarter or more advanced. However, correct grammar depends on using the word that matches the relationship between clauses. In conditional situations, if is simple but powerful and is almost always the right choice. Mixing up connectors that show condition, result, and contrast is a common error in competitive exam English and should be carefully avoided by paying attention to the logical relationship between ideas.

Final Answer:
The completed sentence is was achieved. If you wanted the flame to be bigger, you had to hit the piston again.