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. P: But there is a common thread in their work. Q: In economic jargon it’s 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. S6: Among recent winners of Nobel Prize in Economics, it’s hard to think of one issue which is more topical and relevant to India. Find the correct sequence of P, Q, R and S.