Nucleophilic addition reaction will be most favoured in which among the following compounds?

Reactant $A$ of the above reaction is
$(A)$ + $\begin{matrix} CH_2-OH \\ | \\ CH_2-OH \end{matrix}$ $\xrightarrow{\text{Pyridine}} \begin{matrix} CH_2-O \\ | \\ CH_2-O \end{matrix} > C = O$

Butanone reacts with methyl magnesium bromide to form an addition product $(Z)$. On hydrolysis,$(Z)$ gives:

When liquid $A$ is treated with a freshly prepared ammoniacal silver nitrate solution,it gives a bright silver mirror. The liquid forms a white crystalline solid on treatment with sodium hydrogensulphite. Liquid $B$ also forms a white crystalline solid with sodium hydrogensulphite but it does not give a test with ammoniacal silver nitrate. Which of the two liquids is an aldehyde? Write the chemical equations of these reactions also.

Assertion : Isobutanal does not give iodoform test.
Reason : It does not have $\alpha - $ hydrogen.

Explain the iodoform test.