$A$ $\xrightarrow{K_2Cr_2O_7 / \text{dil. } H_2SO_4} B$ $\xrightarrow{CH_3MgI / H_2O} CH_3-C(CH_3)(OH)-CH_3$. The reactant $A$ is:

Alcohols of low molecular weight are

Explain the types of reactions given by alcohol compounds with examples.

Given below are two statements: One is labelled as Assertion $A$ and the other is labelled as Reason $R$:
Assertion $A$: Alcohols react both as nucleophiles and electrophiles.
Reason $R$: Alcohols react with active metals such as sodium,potassium and aluminum to yield corresponding alkoxides and liberate hydrogen.
In the light of the above statements,choose the correct answer from the options given below:

What is the compound $D$ in the following reaction sequence?
$CH_3CH_2OH$ $\xrightarrow{P + I_2} A$ $\xrightarrow{Mg} B$ $\xrightarrow{HCHO} C$ $\xrightarrow{H_2O} D$

An alcohol $X$ $(C_4H_{10}O)$ on dehydration gives an alkene $(C_4H_8)$ as the major product,which on bromination followed by treatment with $Y$ gives an alkyne $C_4H_6$. The alkyne $C_4H_6$ does not react with sodium metal. What are $X$ and $Y$?