A target is made of two plates, one of wood and the other of iron. The thickness of the wooden plate is $4 \mathrm{cm}$ and that of the iron plate is $2 \mathrm{cm}$. A bullet fired goes through the wood first and then penetrates $1 \mathrm{cm}$ into iron. A similar bullet fired with the same velocity from the opposite direction goes through iron first and then penetrates $2 \mathrm{cm}$ into the wood. If $a_1$ and $a_{\mathrm{2 }}$be the retardation offered to the bullet by wood and iron plates respectively, then

• A

  $a_1=2a_2$
• Correct answer
  B

  $a_2=2a_1$
• C

  $a_1=a_2$
• D

  Data insufficient

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