In the given $TLC$ plate,the distances of spots $A$ and $B$ from the base line are $4 \ cm$ and $6 \ cm$,respectively. The distance moved by the solvent front from the base line is $8 \ cm$. If the $R_f$ value of $B$ is $x \times 10^{-1}$ times that of $A$,find the value of $x$.

• A
  $10$
• B
  $15$
• C
  $20$
• D
  $25$