By current research, aboriginal people first came to the north american continent about when?

By current research, aboriginal people first came to the north american continent about 14,000 to 50,000 years ago. 
The other options show either a very short period of time, or a very long period of time. Even though we don't know the exact time when these people came to North America, we can estimate a particular period during which they decided to settle there. 

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