- Gas-efficient engines uses less gasoline
- Using gasoline requires burning gasoline
- So, gas-efficient engines will burn less gasoline

- Gas-efficient engines will burn less gasoline
- Burning gasoline is a source of air pollution
- Therefore, gas-efficient engines will cause less air pollution

- Gas-efficient engines will cause less air pollution
- An engine is a part of an automobile
- Hence, using a gas-efficient engine in automobiles will cause less air pollution

So, using a gas-efficient engine in automobiles will cause less air pollution
Question: Automobile engines built today are designed to be gas efficient. Gas-efficient engines most likely affect a city by reducing
Context: - Burning gasoline is a source of air pollution 
- Gas-efficient engines uses less gasoline 
- Using gasoline requires burning gasoline 
- An engine is a part of an automobile