Question

To increase the percentage of oxygen in the atmosphere:
A.Plant more trees
B.Kill all living being
C.Cut down trees
D.Protect animal life

Answer 1

Hint: Processes which involve liberation of oxygen gas in the air have to be promoted in order to increase the percentage of oxygen in air. Reforestation, reducing the use of non biodegradable products and reducing the sources of air pollution are some ways to approach it.

Complete answer:
All the matters are made up of particles. The size of particles in a matter is extremely small as a few masses of matter can have millions of tiny particles. A pure matter has all the constituent particles same in their chemical nature or we can say a pure substance consists of a single type of particle. On the other hand, mixtures are constituted by more than one kind of substance. So, air is a matter which is not pure but rather is a mixture of different types of substance such as gases, water vapour, etc.
The composition of air is divided into three categories namely major component, minor component and trace components. Major components of air are composed of nitrogen gas, oxygen gas and water vapour. Minor component of air consists of argon and carbon dioxide gas. Substances like helium, neon, krypton, methane, carbon monoxide, oxide of nitrogen, sulphur dioxide, ammonia, ozone, etc all form the trace component of air.
The percentage compositions of constituents of air are as given below:

Name of the constituent	Its percentage composition in air
Nitrogen gas	\[78.1\% \]
Oxygen gas	\[20.9\% \]
Carbon dioxide gas	\[0.03\% \]
Argon	\[0.9\% \]

Rest trace elements are present in very little composition in air.
Increase in oxygen percentage in the atmosphere can be done by planting more and more trees and reducing the rate at which trees are being cut down as trees and green plants are the absolute source of oxygen in the atmosphere. They do so by the process called photosynthesis in which oxygen is formed as byproduct and released in the atmosphere. Cyanobacteria were the organism which releases oxygen gas for the first time in air.

Thus, the correct option is A.
Note:
Air is a homogeneous mixture and can be separated into its components by fractional distillation process. During fractional distillation, air is subjected to increasing pressure and low temperature, and then allows warming up slowly in the fractional distillation column. Gases get separated at different heights.