Probably because of history relation.
<em>Answer:</em>
<em>C. when the cells engulfed smaller prokaryotic cells</em>
I think the answer would be D
While it is a common belief to people in the ancient civilisations that the Earth is considered as "flat", the contrary idea that the Earth could be considered as "spherical" was first theorised by the famous ancient philosopher Aristotle when he observed that a circular shadow was cast by the moon during a lunar eclipse.
