Two unknown resistances are connected in two gaps of a meter-bridge. The null point is obtained at $40 \mathrm{cm}$ from left end. A $30 \mathrm{\Omega }$ resistance is connected in series with the smaller of the two resistances, the null point shifts by $20 \mathrm{cm}$ to the right end. The value of smaller resistance in $\mathrm{\Omega }$ is

• A

  $12$
• Correct answer
  B

  $24$
• C

  $36$
• D

  $48$

Hence, the correct answer is option: 'B'.