Reordering of sentences (Nobel Prize and externalities): arrange P, Q, R and S to form a coherent paragraph between S1 and S6. S1: The Nobel Prize for Economics in 2018 was awarded to Paul Romer and William Nordhaus for their work in two separate areas: economic growth and environmental economics respectively. S6: Among recent winners of the Nobel Prize in Economics, it is hard to think of one issue which is more topical and relevant to India.

Introduction / Context:
This paragraph is about the work of Paul Romer and William Nordhaus and the economic concept of externalities. The jumbled sentences P, Q, R and S must be arranged to form a logically connected explanation between S1 and S6. To solve this, you need to follow how the passage introduces a common thread in their work and defines key terms before referring back to their contributions.

Given Data / Assumptions:

• S1: The Nobel Prize for Economics in 2018 was awarded to Paul Romer and William Nordhaus for their work in two separate areas: economic growth and environmental economics respectively.
• S6: Among recent winners of the Nobel Prize in Economics, it is hard to think of one issue which is more topical and relevant to India.
• P: But there is a common thread in their work.
• Q: In economic jargon it is termed as externality.
• R: Productive activity often has spillovers, meaning that it can impact an unrelated party.
• S: Romer and Nordhaus both studied the impact of externalities and came up with profound insights and economic models.

Concept / Approach:
The logical development should:

• Introduce the idea that, despite different fields, there is a common thread (P).
• Explain the general concept behind that common thread using an example of spillovers (R).
• Name this concept using the technical term \"externality\" (Q).
• Finally, connect the concept back to Romer and Nordhaus and their work (S), before S6 comments on the relevance.

This structure moves from the prize announcement to concept explanation and then back to the laureates, making the paragraph coherent.

Step-by-Step Solution:
Step 1: After S1, which mentions that the laureates worked in different areas, it is natural to state that there is nevertheless a common thread in their work. P does exactly this. Step 2: Once the idea of a common thread is raised, the passage explains what that thread involves. R introduces the notion of spillovers: productive activity impacting unrelated parties. Step 3: After describing spillovers in simple language, Q names the concept in economic jargon: \"externality\". This is the precise term for the phenomenon described in R. Step 4: Finally, S connects the technical concept back to the laureates: both Romer and Nordhaus studied externalities and created important models. Step 5: Putting this sequence together gives S1 → P → R → Q → S → S6, which is P R Q S.

Verification / Alternative check:
Try starting with R directly after S1. The reader would suddenly see an example of spillovers without understanding why it is being mentioned. Similarly, if Q comes before R, the technical term \"externality\" appears without an illustrative explanation, which is poor expository style. The only order that introduces, explains, labels, and then applies the concept is P R Q S.

Why Other Options Are Wrong:
Option \"P Q R S\": Here the technical term \"externality\" appears before any explanation of spillovers, forcing the reader to deal with jargon without context. Option \"Q S P R\": This begins with jargon, then shifts to Romer and Nordhaus, and only later introduces the example of spillovers, breaking the logical flow. Option \"Q S R P\": The common thread is not introduced first, the technical term comes too early, and the explanation of spillovers appears later, confusing the reader.

Common Pitfalls:
In such conceptual paragraphs, candidates sometimes place the definition (Q) before the illustrative explanation (R), but good writing usually moves from concrete example to abstract label. Also, the sentence that explicitly introduces a \"common thread\" should almost always come immediately after noting differences, as it does in this passage.

Final Answer:
The correct arrangement of the middle sentences is P R Q S, forming a smooth explanation of externalities and the work of Romer and Nordhaus.