|q⟩ Bad Qubits

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Multiple choice
The number operator is N=a†aN = a^\dagger a, with N∣n⟩=n∣n⟩N|n\rangle = n|n\rangle. Summing its diagonal entries over the first three levels, what is ⟨0∣N∣0⟩+⟨1∣N∣1⟩+⟨2∣N∣2⟩\langle 0|N|0\rangle + \langle 1|N|1\rangle + \langle 2|N|2\rangle?