The correct options are:
- Improved engines; engines were improved significantly so that they can have the same power and agility but by using less fuel.
- Cleaner burning fuels; it has been invested a lot of time and money into improving the fuels and making them as effective as possible but also removing or lowering the most dangerous components towards the environment.
- Catalytic converters; this devices for exhaust emission control manage to convert the toxic gasses and polutants into less toxic pollutants by managing to catalyze a redox reaction.
The more carbon dioxide, the more the atmosphere warms up due to the greenhouse effect.
Greenhouse effect- greenhouse gases trap the suns heat making the earth warmer
Western European nations maintained democratic political systems.
During the early earth, when photosynthesis has not yet evolved, there were only negligible amounts of free oxygen in the atmosphere. It was said that the Earth's atmosphere had no free oxygen then. This is about 3.5 billion years ago.
