In each of the following series of compounds or ions, identify those that are isoelectronic (with respect to the valence electrons) and those that are also isostructural: 

(a) [SiO₄]⁴⁻ , [PO₄]³⁻ , [SO₄]²⁻;

(b) CO₂, SiO₂, SO₂, TeO₂, [NO₂]+; 

(c) SO₃, [PO₃]⁻, SeO₃; 

(d) [P₄O₁₂]⁴⁻ , Se₄O₁₂, [Si₄O₁₂]⁸⁻