Which would undergo S_N2 reaction in the following pair and why?

CH₃ –CH₂ –Br undergoes S_N2 reaction. S_N2 reaction readily occurs in substituted primary alkyls. S_N2 is substitution nucleophilic bimolecular reaction. In this reaction the nucleophile attacks the alkyl bromine from back, which leads to simultaneous, bond breaking between the C and –Br and bond forming between –C- and nucleophile. This is possible because:

a. It does not provide any stearic hindrance for back attack.

b. Ethyl cation is a primary carbocation hence not stable, so will not be readily formed.

In case of t-butlybromide, t-butyl carbocation is highly stable and the back attack by nucleophile is not possible due to stearic hindrance hence it will undergo S_N1 reaction readily then S_N2.