Answer: A, C, D
SN2 is bimolecular: rate =k[CHX3Br][OHX−], so doubling either concentration doubles the rate. SN1 rate =k[(CHX3)X3CBr], independent of [OHX−] but proportional to the halide concentration. The methyl cation is highly unstable, so the SN2 path for CHX3Br does not involve it.