Numerical answer
The four sentences below, labelled 1, 2, 3 and 4, can be arranged into a coherent paragraph. Decide the correct order.
1. Lichens are not single organisms but a partnership between a fungus and either an alga or a cyanobacterium, living so closely intertwined that the pair was long mistaken for one species.
2. This composite biology allows lichens to survive in environments, bare rock, arctic tundra, desert crust, that would be lethal to either partner alone.
3. The fungal partner supplies structure and protection, while the photosynthetic partner supplies sugars manufactured from sunlight.
4. It is this extraordinary hardiness that has made lichens useful as indicators of air quality, since they absorb pollutants directly from the atmosphere and respond visibly to contamination.
1. Lichens are not single organisms but a partnership between a fungus and either an alga or a cyanobacterium, living so closely intertwined that the pair was long mistaken for one species.
2. This composite biology allows lichens to survive in environments, bare rock, arctic tundra, desert crust, that would be lethal to either partner alone.
3. The fungal partner supplies structure and protection, while the photosynthetic partner supplies sugars manufactured from sunlight.
4. It is this extraordinary hardiness that has made lichens useful as indicators of air quality, since they absorb pollutants directly from the atmosphere and respond visibly to contamination.
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Answer: 1324
Sentence 1 introduces what lichens actually are (a fungus-alga/cyanobacterium partnership). Sentence 3 explains the division of labour between the two partners. Sentence 2 states the resulting benefit (survival in extreme environments). Sentence 4 builds on that hardiness ('It is this extraordinary hardiness...') to explain lichens' practical use as pollution indicators. The order 1→3→2→4 moves from definition to mechanism to benefit to application.
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