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**Educated** Well i'm not good at explaining things but I'll try to explain as best as I can:

It's the x^3 that tells us which answer it is. It tells us that as x goes to positive infinity, so does y. But if it were -(x^3), then this would tell us that as x goes to positive infinity, y goes to negative infinity.

Since the x^3 is positive, x would have to be larger than the largest root because it goes to infinity, and so $\displaystyle x > 4$. When we know this, we know the rest.

Sorry I'm not better at my explanations.