Which one of the structures shown is NOT superimposable on its mirror image? Consider configuration only (treat rings as planar).

(A) is trans-1,2-dimethylcyclohexane: the two stereocentres have the same descriptor, (1R,2R) or (1S,2S), there is no mirror plane, and the compound is chiral. (B) cis-1,4-dimethylcyclohexane has a mirror plane through C1 and C4 and no stereocentres. (C) trans-1,3-dimethylcyclobutane has a mirror plane through C1 and C3. (D) is (2R,4S)-2,4-dibromopentane, the meso form, with a mirror plane through C3. Only (A) is chiral.