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One solution contains the 1st required ion, and the other solution contains the 2nd required ion.
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This question is asking us to identify a chemical reaction that forms an insoluble sulfate salt.
And if you were to look at ammonium, carbonate, carbonates are generally insoluble, But ammonium.
Why is NaCl insoluble in water? The water-insoluble sulfates are also insoluble in dilute acids.