Question

Why is the answer D?

Answer 1

Step-by-step explanation:

Inflection points are where a function changes concavity (the second derivative changes signs).  At x = 4, f"(x) goes from negative to positive, so that's an inflection point.  However, at x = 8, we don't know if f"(x) changes signs or not.  So we can't say that that's an inflection point.