Which of the following is the correct order of decreasing $S_{N}2$ reactivity? ($X=\text{a halogen}$)

The $S_{N}2$ reaction involves a backside attack by the nucleophile on the carbon atom bonded to the leaving group. This process is highly sensitive to steric hindrance.Primary alkyl halides (${\text{RCH}}_2\text{X}$) have the least steric hindrance, making them most reactive.Secondary alkyl halides (${\text{R}}_2\text{CHX}$) are less reactive.Tertiary alkyl halides (${\text{R}}_3\text{CX}$) are the least reactive (almost inert to $S_{N}2$) due to extreme steric crowding.Order: $1^{\circ }>2^{\circ }>3^{\circ }$.