121- The set of lengths of inflection points of the curve $y = x|x^2 - 4x|$ is which of the following?
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(1) $\left\{\dfrac{4}{3}\right\}$ \hfill (2) $\left\{0, \dfrac{4}{3}, 4\right\}$ \hfill (3) $\left\{\dfrac{4}{3}, 4\right\}$ \hfill (4) $\left\{0, \dfrac{4}{3}\right\}$
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