What is the product $X$ in the following reaction?
$CH_3CHO + CH_2(COOH)_2 \xrightarrow{\Delta} X$

Identify the compound $(B)$ in the following reaction sequence:
$2\text{-formylmethylbenzoic acid}$ $\xrightarrow{NaBH_4} (A)$ $\xrightarrow{H^{\oplus}, \Delta} (B) \text{ (Cyclic)}$

Reduction of carboxylic acids gives

$A$ fruity smell is produced by the reaction of $C_2H_5OH$ with

Oxalic acid reacts with concentrated $H_2SO_4$ to produce a mixture of two gases. When this mixture is passed through caustic potash,one of the gases is absorbed. What substance is formed by the absorption of the gas in caustic potash?

The relative reactivities of acyl compounds towards nucleophilic substitution are in the order of