Assuming That the Proposed Reactants Are Mixed in Stoichiometric Ratios

Multiple Choice

Assuming that the proposed reactants are mixed in stoichiometric ratios, and at moderate concentrations, which of the following is least likely to represent an observable (e.g., gas evolution or precipitation) reaction?

A) Ca²⁺ + 2 Cl- + 2 K⁺ + CO₃^2- → ?
B) 2 Na⁺ + CO₃^2- + 2 HC₂N₃O₂ → ?
C) H₂S + Cu²⁺ + SO₄^2- → ?
D) H₂SO₃ + Li⁺ + Cl- → ?
E) Pb²⁺ + 2 NO₃^- + 2 Na⁺ + 2 OH- → ?