A two-dimensional harmonic oscillator has energy $E=$ $\hbar \omega_{0}\left(n_{x}+n_{y}+1\right),$ where $n_{x}$ and $n_{y}$ are integers beginning with zero. ( $a$ ) Justify this result based on the energy of the one-dimensional oscillator. (b) Sketch an energy-level diagram similar to Figure 5.21 , showing the lowest 4 energy levels. For each level, show the value of $E$ (in units of $\hbar \omega_{0}$ ), the quantum numbers $n_{x}$ and $n_{y},$ and the degeneracy. (c) Show that the degeneracy of each level is equal to $n_{x}+n_{y}+1$