But-1-ene is treated with $HBr$ (no peroxide) to give A. A is heated with alcoholic KOH to give B as the major product. B is treated with $O X_{3}$ and then $Zn / H X_{2} O$ to give C. Considering only the major route, C is

Answer: D

Markovnikov addition gives A = 2-bromobutane. Saytzeff elimination gives B = but-2-ene. Reductive ozonolysis cleaves

CH X_{3} - CH = CH - CH X_{3}

into two molecules of

CH X_{3} CHO

, so C is ethanal.