An alkene B gives the compound shown and ethanal on ozonolysis with reductive work-up. Which alkene is B?

The compound shown is propanone. Joining the carbonyl carbon of propanone to the carbonyl carbon of ethanal through a double bond gives 2-methylbut-2-ene, $\mathrm{CH}{}_3\mathrm{CH}=\mathrm{C}(\mathrm{CH}{}_3){}_2$. The 2-methylbut-1-ene and 3-methylbut-1-ene isomers would give methanal, and pent-2-ene gives propanal.