Which of the following is least soluble in water?

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$K_2[Cd(CN)_4] + H_2S \longrightarrow CdS \downarrow + 2KCN + 2HCN$

Which of the following equations represents the solubility product $(K_{sp})$ of $As_2S_3$?

If the solubility product of $CaSO_4$ is $2.5 \times 10^{-5}$,then its solubility will be .......

$A$ white powder soluble in $NH_4OH$ but insoluble in water is:

$K_{sp}$ for $BaSO_4$ at $25 \, ^oC$ is $1.1 \times 10^{-10}$. To make the new solubility equal to $1.1 \times 10^{-8} \, M$,it is necessary to use a solution of $Na_2SO_4$ of the following concentration $....... \ M$