Which one is the outermost layer of atmosphere?

1. Ionosphere
2. Stratosphere
3. Exosphere
4. Troposphere

Which of the following is not a reason for global warming?

1. Rise in the sea level
2. Ozone depletion
3. Deforestation
4. Burning of fossil fuel

Which of the following is not a greenhouse gas?

1. Ozone
2. CO₂
3. Helium
4. Methane

Wheat is one of the most important staple foods consumed globally, with the USA producing 8% of the world's total production. But the recent studies show that the wheat production is declining here due to adversely of climate.

As a geographical point of view can you state the specific reason to explain the fact?