I think that the best phrase to fit into the blank is a "scientific theory".
A scientific theory is developed through experiments which use the scientific method, and a theory can be improved if additional data is available or if people provide suggestions which make the theory fit the data better.
<span>The element Carbon is the </span>building block for all<span> known living things. That is why, in some science-fiction movies you hear the term, Carbon-based units.</span>
They have different genes.
