Which of the following statements about the spectra and magnetism of trivalent lanthanide ions are correct?

1. AThe spin-only formula reproduces the observed moments of all $\mathrm{Ln}{}^{3+}$ ions accurately.
2. BTheir colours arise from f-f transitions.
3. CTheir absorption bands are sharper than d-d bands because the 4f orbitals are shielded by the filled 5s and 5p subshells.
4. D$\mathrm{Lu}{}^{3+}$ is colourless and diamagnetic.