$D$-Mannose $\overset{HO^{-}}{\longleftrightarrow}$ $D$-Glucose $\overset{HO^{-}}{\longleftrightarrow}$ $(A)$.
Product $(A)$ of the above reaction is:

The given structures are $C_n$ epimers. What is $n$?

The two functional groups present in a typical carbohydrate are:

$A$ nucleoside on hydrolysis gives:

$L$-isomer of tetrose $X$ $(C_4H_8O_4)$ gives positive Schiff's test and has two chiral carbons. On acetylation,$X$ yields triacetate. $X$ also undergoes the following reactions:
$X$ $\xrightarrow{HNO_3} A$ $\xrightarrow{NaBH_4} \underset{\text{Chiral compound}}{B}$
$X$ is $.......$.

$D$-Glucose exists in $x$ different forms. The value of $x$ (stereoisomer) is