The order of reactivity for the S_N2 reaction involving replacement of-X by a nucleophile in alkyl halide is 

• A

  ${\mathrm{CH}}_3<1^{\circ }<2^{\circ }$

• B

  ${\mathrm{CH}}_3>1^{\circ }>2^{\circ }$

• C

  ${\mathrm{CH}}_3<1^{\circ }>2^{\circ }$

• D

  ${\mathrm{CH}}_3>1^{\circ }<2^{\circ }$