L-isomer of tetrose $\mathrm{X}\left({\mathrm{C}}_{\mathrm{4}}{\mathrm{H}}_{\mathrm{8}}{\mathrm{O}}_{\mathrm{4}}\right)$ gives positive Schiff’s test and has two chiral carbons. On acetylation, ‘X’ yields triacetate. ‘X’ also undergoes the following reactions $\text{'}\mathrm{A}\text{'}\stackrel{{\mathrm{HNO}}_3}{\leftarrow }\text{'}\mathrm{X}\text{'}\stackrel{{\mathrm{NaBH}}_4}{\to }\underset{\mathrm{Chiral}\mathrm{compound}}{\mathrm{B}}$ 'X' is