L-isomer of tetrose $\mathrm{X}\left(\mathrm{C}_{4} \mathrm{H}_{8} \mathrm{O}_{4}\right)$ gives positive Schiff's test and has two chiral carbons. On acetylation, '$\mathrm{X}$' yields triacetate. '$\mathrm{X}$' also undergoes following reactions

'$\mathrm{X}$' is