Hypothetical relative rate constants for SN2 reaction of $\mathrm{RBr}$ with iodide in acetone (ethyl bromide $=1.00$) are: $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{Br}$ 1.00; $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2\mathrm{Br}$ 0.40; $(\mathrm{CH}{}_3){}_2\mathrm{CHCH}{}_2\mathrm{Br}$ 0.030; $(\mathrm{CH}{}_3){}_3\mathrm{CCH}{}_2\mathrm{Br}$ $1.0\times 10^{-5}$. From these data, isobutyl bromide is faster than neopentyl bromide by a factor of, and the trend shows that

1. A$3\times 10^3$; beta-branching has little effect until the carbon becomes tertiary
2. B$3\times 10^3$; isobutyl bromide is slower because it has fewer beta-methyl groups
3. C$30$; each additional beta-methyl group slows the reaction by the same factor
4. D$3\times 10^3$; each additional beta-methyl group slows the reaction, the third one by far the most