SN1 vs. SN2 - 2 | Alkyl Halides - Nucleophilic Substitutions

Organic Chemistry 1 - SN1 vs. SN2 - 2

What is the likely mechanism of nucleophilic substitution for the following alkyl halides?

Methyl and primary alkyl halides: only SN2

Secondary alkyl halides: SN1 and SN2

Tertiary alkyl halides: only SN1