Which of the two stereoisomers of $4-t-$butylcyclohexyl iodide $(^{127}I^-)$ will undergo $S_N2$ substitution with $^{128}I^-$ faster,and why?

• A
  $A$ will react faster because it is the more stable of the two isomers.
• B
  $A$ will react faster because it will yield a more stable product,and the transition state for both reactions is of the same energy.
• C
  $A$ will react faster because the approach of $^{128}I^-$ can occur unhindered.
• D
  $B$ will react faster because it is less stable than $A$,and the transition state for both reactions is of the same energy.