At $518^{\circ}C$, the rate of decomposition of a sample of gaseous acetaldehyde, initially at a pressure of $363 $ Torr, was $1.00 \; Torr\;s^{-1}$ when $5\%$ had reacted and $0.5\; Torr\; s^{-1}$ when $33\%$ had reacted. The order of the reaction is :
