The four sentences (labelled 1, 2, 3, 4) below, when properly sequenced would yield a coherent paragraph. Decide on the proper sequencing of the order of the sentences and key in the sequence of the four numbers as your answer:

1. It also has four movable auxiliary telescopes 1.8 m in diameter.
2. Completed in 2006, the Very Large Telescope (VLT) has four reflecting telescopes, 8.2 m in diameter that can observe objects 4 billion times weaker than can normally be seen with the naked eye.
3. This configuration enables one to distinguish an astronaut on the Moon.
4. When these are combined with the large telescopes, they produce what is called interferometry: a simulation of the power of a mirror 16 m in diameter and the resolution of a telescope of 200 m.

The correct answer is 2143. This means the sentences should be read in the order: Sentence 2 → Sentence 1 → Sentence 4 → Sentence 3.

To understand why this is the correct sequence, let us analyze the logical and thematic flow of each sentence and how they connect to one another.

Step 1: Identify the Opening Sentence
Sentence 2 is clearly the best opening sentence. It introduces the main subject — the Very Large Telescope (VLT) — and provides foundational facts: its completion date (2006), its primary configuration (four reflecting telescopes, 8.2 m in diameter), and its extraordinary capability (observing objects 4 billion times weaker than what the naked eye can see). A good paragraph opener introduces the topic, and Sentence 2 does exactly that. No other sentence can logically precede it because they all assume the reader already knows what is being discussed.

Step 2: Identify the Second Sentence
Sentence 1 follows naturally after Sentence 2. Notice the critical word "also" at the beginning: "It also has four movable auxiliary telescopes 1.8 m in diameter." The word "also" is a cohesive device — it signals an addition to something already mentioned. Since Sentence 2 describes the primary four large telescopes, Sentence 1 logically adds the next piece of information: the four smaller auxiliary (helper) telescopes. The pronoun "It" refers back to the VLT introduced in Sentence 2, further confirming this order.

Step 3: Identify the Third Sentence
Sentence 4 comes next. It begins with "When these are combined with the large telescopes..." — the phrase "these" refers directly to the auxiliary telescopes just described in Sentence 1, and "the large telescopes" refers back to the primary telescopes from Sentence 2. Sentence 4 then explains the outcome of combining both sets of telescopes: a phenomenon called interferometry, which simulates the power of a 16 m mirror and the resolution of a 200 m telescope. This sentence builds upon the two preceding ones and introduces the climactic technical capability.

Step 4: Identify the Closing Sentence
Sentence 3 is the logical conclusion: "This configuration enables one to distinguish an astronaut on the Moon." The phrase "This configuration" refers directly back to the interferometric setup described in Sentence 4. Sentence 3 provides a vivid, concrete, relatable illustration of just how powerful that combined configuration is. It acts as the memorable, punchy closing statement that drives home the extraordinary resolving power of the VLT system.

Summary of the Logical Chain:

  Sentence 2 → Introduces the VLT and its four main large telescopes.
      ↓
  Sentence 1 → Adds the four auxiliary telescopes ("also" signals addition).
      ↓
  Sentence 4 → Explains what happens when both sets of telescopes are combined (interferometry).
      ↓
  Sentence 3 → Gives a vivid real-world example of what this configuration can achieve.

The key to solving such sentence-ordering questions is to look for cohesive devices — words like "also," "these," "this," and pronouns like "it" — that explicitly link one sentence to another. In this paragraph, each sentence hands off a specific idea to the next, creating a smooth, logical progression from introduction → addition → combination → outcome.

Therefore, the correct sequence is 2 → 1 → 4 → 3, confirming the answer 2143.