A. cats have changed little in the millions of years that it has existed. --->Pronoun error. "Cats" is plural whereas "it" is singular. INCORRECT
B. the cat changed little in the millions of years of its existence. --->We need to show continuity here by saying that since millions of years, there has been little change. The verb "has" is missing. The current sentence changes the meaning. INCORRECT
C. the cat has changed little in the millions of years it was
...
B. the cat changed little in the millions of years of its existence. --->We need to show continuity here by saying that since millions of years, there has been little change. The verb "has" is missing. The current sentence changes the meaning. INCORRECT
C. the cat has changed little in the millions of years it was
...
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