Let Y = ƒ(X) Be a Differentiable Function for Which

Multiple Choice

Let y = ƒ(x) be a differentiable function for which the graph of its derivative, ƒ', is given below:
At what x-value(s) , if any, does the graph of ƒ have a horizontal tangent?
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A) x = 0 only
B) x = -3 only
C) x = 3 only
D) x = -3 and x = 3
E) None