The source of energy that can be used to reduce the negative impact of non-renewable sources of energy replacing these sources is called an alternative source of energy. Its examples are:

1. Biofuel
2. Nuclear energy

Any three advantages of hydropower are:

1. It is a renewable source of energy.
2. It is eco-friendly.
3. It is cheaper in the long-term.

Any two reasons for the statement 'hydropower is better than fossil fuel.' are:

1. Hydropower is a renewable source but fossil fuel is non-renewable.
2. Hydropower is eco-friendly but fossil fuel pollutes the environment.

The liquid formed of fossil fuels such as petrol, diesel, and so on is called mineral oil. Any three advantages are:

1. It is cheaper for long-term production.
2. It can be easily transported.
3. It can be used to generate electricity.

The fuel formed from the fossils of plants and animals buried under the earth's crust for a long time is called fossil fuel. Any two examples are:

1. Coal
2. Petroleum products

Any three conditions required for the thermonuclear fusion reaction in the sun are:

1. In the sun, there is a sufficient amount of hydrogen gas.
2. There is a very high temperature in the sun which is essential for the formation of free protons.
3. There is necessary pressure for the combination of free protons.

As we know that the sun provides both heat and light energy for all living beings. It is the main source of energy as it supplies energy for all organisms directly or indirectly. It is the storehouse of energy. It also supports the existence of other sources of energy like fossils, solar energy, hydroelectricity, bio-fuel, and so on. So, we can say that all the energy sources are the outcome of solar energy. So, the sun is the main source of energy.

The energy sources that are continuously produced in nature and are inexhaustible are called renewable sources of energy. Some examples are:

1. Solar energy
2. Tidal energy

The type of energy sources that get depleted and cannot be replaced quickly when exhausted are called non-renewable sources of energy. Any 2 examples are:

1. Fossil fuel
2. Natural gas

The primary sources of energy are the sources of energy that are used in the same form as they are available in nature.

The different sources from which we can obtain various types of energy are meant by sources of energy. The 2 examples are:

1. Sun --> Perpetual source
2. Fossil fuel --> Non-renewable source