Which of the following is the correct order of decreasing SN^2...

Options

1. A$\mathrm{RCH}_2 \mathrm{X}>\mathrm{R}_3 \mathrm{CX}>\mathrm{R}_2 \mathrm{CHX}$
2. B$\mathrm{RCH}_2 \mathrm{X}>\mathrm{R}_2 \mathrm{CHX}>\mathrm{R}_3 \mathrm{CX}$
3. C$\mathrm{R}_3 \mathrm{CX}>\mathrm{R}_2 \mathrm{CHX}>\mathrm{RCH}_2 X$
4. D$\mathrm{R}_2 \mathrm{CHX}>\mathrm{R}_3 \mathrm{CX}>\mathrm{RCH}_2 \mathrm{X}$ $(\mathrm{X}=$ a halogen $)$