Which of the following alkenes give only one organic product on ozonolysis followed by $\mathrm{Zn}/\mathrm{H}{}_2\mathrm{O}$?

A symmetrical alkene gives two identical carbonyl molecules. Hex-3-ene gives $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CHO}$, 2,3-dimethylbut-2-ene gives $\mathrm{CH}{}_3\mathrm{COCH}{}_3$ and but-2-ene gives $\mathrm{CH}{}_3\mathrm{CHO}$. 2-Methylbut-2-ene is unsymmetrical and gives both ethanal and propanone.