When a plant self-fertilizes generation after generation, and always shows the same phenotype for the characteristic being considered, then the plant is called

When plants are allowed to self-fertilize generation after generation, the exact same physical traits appears without any change. Such plants are called pure line or true-breeding. For example, tall plants always produce tall plants and true-breeding dwarf plants always produce dwarf plants on selfing.  

coal is not sustainable because it will one day run out.