\textbf{121.} For which domain, the function $f(x) = x^3 e^{-x}$ is increasing and its graph is concave upward?
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(1) $(0, 3-\sqrt{3})$ \hfill (2) $(3-\sqrt{3}, 3)$ \hfill (3) $(3, 3+\sqrt{3})$ \hfill (4) $(3+\sqrt{3}, +\infty)$
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