Reductive ozonolysis of which alkene gives propanone as the only organic product?

$(\mathrm{CH}{}_3){}_2\mathrm{C}=\mathrm{C}(\mathrm{CH}{}_3){}_2$ has two identical $(\mathrm{CH}{}_3){}_2\mathrm{C}=$ ends, so cleavage gives two molecules of $(\mathrm{CH}{}_3){}_2\mathrm{C}=\mathrm{O}$. 2-Methylbut-2-ene gives propanone plus ethanal, 2-methylpropene gives propanone plus methanal, and 2,3-dimethylbut-1-ene gives propanone plus a different ketone.