(i). A good source of energy is the one which would do a large amount of work per unit volume or mass.
When we want to use something, we give it input and we desire considerable output from that thing and of course we want large amount of output or work done by that thing not a small amount of output or work done.
(ii). Any source of energy we use to do work is consumed and cannot be used again.
When we use some energy, it is in the usable form and when we apply it some energy is used and some energy is dissipated to the surroundings but the amount of energy we used is fully gone with the law of conservation of mass.
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