In Searle's method to find Young's modulus of a wire, when a force of $1 \cdot 5 \mathrm{~kg}$ -wt is applied at its free end, the length of wire is ' $a$ '. When force of $2 \cdot 5$ kg-wt is applied, the length of wire is 'b'. What would be its original length?

• A

  $\mathrm{b}-\mathrm{a}$
• B

  $\frac{\mathrm{b}-\mathrm{a}}{4}$
• Correct answer
  C

  $2 \cdot 5 \mathrm{a}-1 \cdot 5 \mathrm{~b}$
• D

  $2 \cdot 5 \mathrm{~b}-1 \cdot 5 \mathrm{a}$

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