Ecosystem

The correct option is (A): Gas - Oxygen; % in inhaled air - 21%; % in exhaled air - 16%.

The correct option is (B): Charles Elton.

The correct option is (B): Environment.

The correct option is (D): Giraffe - Secondary consumer.

1. Freshwater on Earth:
Only about 2.5\% of Earth's water is freshwater, which includes water in rivers, lakes, glaciers, and underground aquifers. The remaining 97.5\% is saltwater found in oceans. 2. Availability of freshwater:
Of the 2.5\% of freshwater, a large portion is locked in glaciers and ice caps, and only a small fraction is available for human use in rivers and lakes.

1. Abiotic factors:
Abiotic factors are non-living components of an ecosystem that influence the living organisms. They include physical factors such as light, temperature, soil, and water. These factors are essential for the survival and growth of organisms. 2. Biotic vs. Abiotic:
Biotic factors are related to living organisms (like predation, competition, and reproduction), while abiotic factors are physical or chemical components (like light, temperature, and water).