Numerical answer
The four sentences below, when properly sequenced, form a coherent paragraph. Decide on the correct order.
1. Yet extracting drinkable water from a comet is not a matter of simply melting the ice, since much of it is bound so tightly to dust and organic compounds that separating the two is itself a demanding engineering problem.
2. Comets, long studied chiefly as scientific curiosities, are now drawing interest from a different quarter: engineers evaluating their ice as a future source of water for orbital refuelling stations.
3. If that separation problem can be solved economically, a single modest comet could supply more water than has ever been launched from Earth across the entire history of spaceflight combined.
4. This interest stems from a simple calculation: hauling water up from Earth's gravity well is far costlier, per kilogram delivered to orbit, than capturing water already present in space.
1. Yet extracting drinkable water from a comet is not a matter of simply melting the ice, since much of it is bound so tightly to dust and organic compounds that separating the two is itself a demanding engineering problem.
2. Comets, long studied chiefly as scientific curiosities, are now drawing interest from a different quarter: engineers evaluating their ice as a future source of water for orbital refuelling stations.
3. If that separation problem can be solved economically, a single modest comet could supply more water than has ever been launched from Earth across the entire history of spaceflight combined.
4. This interest stems from a simple calculation: hauling water up from Earth's gravity well is far costlier, per kilogram delivered to orbit, than capturing water already present in space.
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Answer: 2413
Sentence 2 introduces the new topic (engineers eyeing comet ice). Sentence 4 explains why (the cost calculation against Earth-launched water) — logically following 'this interest' from sentence 2. Sentence 1 introduces the complication ('Yet...') that separating ice from dust is hard. Sentence 3 concludes with the payoff ('If that separation problem can be solved...') referring back to sentence 1's problem. Order: 2-4-1-3.
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