The sun, or plants
The Sun!
Hydrogen (H)
carbon
The sun.
The Sun
The original source of almost all energy in an ecosystem is the sun.
If by "world" you mean "planet Earth", that would be the Sun.
because plants get their energy from it and everything else gets their energy from plants
That means that we - planet Earth, and all living beings on Earth - get most of our energy, directly or indirectly, from the Sun.
They are because they polute the air and water of the earth.
water and electricity
Mars is the closest planet to the sun, and receives the most energy.