The following results are obtained when an aqueous solution of cation (green spheres) and anion (orange spheres) is mixed: 
Which of the following pairs of cation and anion are consistent with what has been observed? Explain
Cations: K⁺, Cu⁺, Pb²⁺, Co³⁺Anions: NO₃⁻, Cl⁻, S^2–, AsO₄³⁻

• A. The pair of cation and anion consistent with the observation is K₂S because it is soluble in water.
• B. The pair of cation and anion consistent with the observation is PbS because it is insoluble in water.
• C. The pair of cation and anion consistent with the observation is CoAsO₄ because it is insoluble in water.
• D. The pair of cation and anion consistent with the observation is Cu₂S because it is insoluble in water.
• E. The pair of cation and anion consistent with the observation is Co₂S₃ because it is insoluble in water.

Answer: (D)