L-isomer of tetrose X ( C _ 4 H _ 3 O _ 4 ) gives positive Schiff's test and has two…

L-isomer of tetrose X \(\left(\mathrm{C}_{4} \mathrm{H}_{3} \mathrm{O}_{4}\right)\) gives positive Schiff's test and has two chiral carbons. On acetylation, ‘X’ yields triacetate. ‘X’ also undergoes following reactions              (Jee Main 2023)
 \(A \stackrel{\mathrm{FenO}_{3}}{\longleftarrow} X \xrightarrow{\mathrm{MaseL}_{4}} \underset{\text { Conalerquend }}{B}\)
 X is