While hydrogen energy holds promise, the fact is that hydrogen is an energy not a primary energy

The right answer is energy carrier / not a primary energy source

Hydrogen is not strictly speaking a source of energy, but a vector, just like electricity: it is used to transport the energy produced by a primary source (oil, uranium) to the users. Used in a fuel cell, its only waste is water.

Answer;

Carrier; source

While hydrogen energy holds promise, the fact is that hydrogen is an energy carrier, not a primary energy source.

Explanation;

Hydrogen can be considered as a clean energy carrier similar to electricity.

Hydrogen can be produced from various domestic resources such as renewable energy and nuclear energy.

It is considered as a secondary source of energy, commonly referred to as an energy carrier. Energy carriers are used to move, store and deliver energy in a form that can be easily used.

d. planting trees

the trees take in carbon to make oxygen.

the statements that apply here about binary star system are

"have stars that might appear to wobble"

"often have one star that is brighter than other"

a binary star is a star system consisting of two stars revolving around their common barycenter. systems of two or more stars are called multiple star systems.