In the following passage about 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. When the kerosene burned, the fuel in the tank below would be under pressure, and __________ a small circular flame would appear in the burner. In this question you must choose the correct adverb to complete the phrase and __________ a small circular flame would appear in the burner.

Introduction / Context:
This question again comes from the same passage that explains how a kerosene pump stove works. Here the focus is on the correct use of an adverb to describe how a small circular flame appears in the burner when pressure builds up. The question checks whether the learner can distinguish between adjective and adverb forms and select the one that fits grammatically and logically in the sentence.

Given Data / Assumptions:

• The fuel in the tank is under pressure when kerosene burns.
• After this, a small circular flame appears in the burner.
• The sentence part being tested is and __________ a small circular flame would appear in the burner.
• Available options are slow, slowly, slower, and slowest.
• The sentence describes the manner in which the flame appears, not its degree of slowness compared to something else.

Concept / Approach:
English uses adjectives to describe nouns and adverbs to describe verbs, adjectives, or other adverbs. In this sentence, appear is the main verb, and we need a word that tells us how the flame would appear. Therefore, we require an adverb of manner. Slowly is the standard adverb form of slow and fits naturally after the conjunction and in this structure. The comparative form slower and the superlative slowest indicate comparison or ranking, which is not needed here.

Step-by-Step Solution:
Step 1: Identify the verb in the clause, which is would appear. Step 2: Decide what type of word is missing. Since we want to describe the manner of appearance of the flame, we need an adverb. Step 3: Examine the options and classify them. Slow is an adjective, slowly is an adverb, slower is comparative degree, and slowest is superlative degree. Step 4: Substitute each option mentally into the sentence and check grammatical correctness and natural usage. Step 5: Conclude that only slowly yields a correct and natural sentence: and slowly a small circular flame would appear in the burner.

Verification / Alternative check:
You can cross check by converting the sentence structure slightly: A small circular flame would appear slowly in the burner. In this reordered version, it is even clearer that slowly modifies the verb would appear. If you try to use slow or slower in this structure, the result is ungrammatical or odd. This confirms that slowly is the correct choice.

Why Other Options Are Wrong:
slow: This is an adjective typically used before a noun, as in a slow process. It does not fit directly before a because we already have the article and a noun phrase a small circular flame. slower: This comparative form is used when comparing two speeds, for example slower than before. The passage does not compare different speeds of the flame. slowest: This superlative form refers to one item among many, as in the slowest method, and has no contextual support in the passage.

Common Pitfalls:
Learners often confuse adjective and adverb forms when they see familiar base words like slow. They may also overlook the structure of the sentence and choose a degree form like slower simply because they recognise it. The key is to look at the verb would appear and ask what kind of word normally follows. In descriptive passages about processes and machines, adverbs such as slowly, gradually, or rapidly are frequently used to show how changes happen over time.

Final Answer:
The correct completion is and slowly a small circular flame would appear in the burner.