Use obstinacy in a sentence about a two year old

Hi, I hope these sentences can help you with your assignment:

1.       No matter how much we explain to a 2-year old kid, his obstinacy is something natural to such age.

2.       Obstinacy may start as early as two years old, as parents we must be able to intervene and handle it correctly. 

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."

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