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Here are the rewritten sentences: 1. Cars emit greenhouse gases, which affects air quality. If cars didn't produce greenhouse gases, the air quality wouldn't be so bad. (Alternative: The emission of greenhouse gases by cars is the reason for poor air quality.) 2. (No changes needed, as the sentence already conveys the intended meaning.) 3. If we recycle more, we will send less waste to landfills. The more we recycle, the less waste we will send to landfills. (Alternative: The more we recycle, the less waste will be sent to landfills.) 4. We should pack reusable bags in case the store doesn't offer any. (Alternative: In case the store doesn't offer any bags, we should pack reusable ones.) 5. If she works hard, she will get a high salary. The harder she works, the higher her salary will be. (Alternative: The more effort she puts in, the higher her salary will be.) 6. People don't understand him because he doesn't speak clearly. If he spoke more clearly, people would understand him. (Alternative: His unclear speech is the reason people don't understand him.) 7. If you use much electricity, your bill will be high. The more electricity you use, the higher your bill will be. (Alternative: The more electricity you consume, the higher your bill will be.) 8. The car breaks down so often because you don't take good care of it. If you took better care of the car, it wouldn't break down so often. (Alternative: The car's frequent breakdowns are a result of your neglect.) 9. She studies hard, she will get good grades. The harder she studies, the better her grades will be. (Alternative: Her hard work will result in good grades.) 10. You rest much, you will feel better. The more you rest, the better you will feel. (Alternative: The more rest you get, the better you'll feel.) 11. The weather is warmer, I feel better. The warmer the weather, the better I feel. (Alternative: As the weather warms up, I feel better.) 12. We leave early, we will arrive soon. The earlier we leave, the sooner we will arrive. (Alternative: An early departure will result in an early arrival.) 13. We have much knowledge, we become wise. The more knowledge we have, the wiser we become. (Alternative: The more we learn, the wiser we become.)