Arrange the acidity of the given four compounds in decreasing order:
$(I)$ $CH_3COCH_2COCH_3$
$(II)$ $CH_3COCH_3$
$(III)$ $CH \equiv CH$
$(IV)$ $CH_3CHO$
Select the correct answer from the codes given below:

Give plausible explanation for each of the following:
$(i)$ Cyclohexanone forms cyanohydrin in good yield but $2,2,6-$trimethylcyclohexanone does not.
$(ii)$ There are two $-NH_2$ groups in semicarbazide. However,only one is involved in the formation of semicarbazones.
$(iii)$ During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst,the water or the ester should be removed as soon as it is formed.

The product $(A)$ of the reaction is:

In the following reaction sequence,the compound $J$ is an intermediate.
$I$ $\xrightarrow{(CH_3CO)_2O / CH_3COONa} J$ $\xrightarrow[(ii) \text{ anhyd. } AlCl_3]{(i) H_2, Pd/C, (ii) SOCl_2} K$
$J \left( C_9H_8O_2 \right)$ gives effervescence on treatment with $NaHCO_3$ and positive Baeyer's test.
$1.$ The compound $K$ is
$2.$ The compound $I$ is
Give the answer for question $1$ and $2.$

The correct acidity order of the following compounds is:

The acidity of compounds $I-IV$ in water follows the order:
$I.$ ethanol
$II.$ acetic acid
$III.$ phenol
$IV.$ acetonitrile