$\begin{array}{1 1}(A)\;41mJ\\(B)\;29mJ\\(C)\;20.5mJ\\(D)\;\text{None of above}\end{array} $

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As the elastic energy of string E =$\large\frac{1}{2}$$\times$ force$\times$ extension

Work of string=$E_2-E_1$

$\Rightarrow \large\frac{1}{2}$$\times 10\times 10\times .7\times 10^{-3}$$-\large\frac{1}{2}$$\times 4\times 10\times 10^{-3}\times .3$

$\Rightarrow \large\frac{1}{2}$$\times (7-1.2)\times 10^{-2}$

$\Rightarrow \large\frac{5.8}{2}$$\times 10^{-2}$

$\Rightarrow 2.9\times 10^{-2}J$

$\Rightarrow 29mJ$

Hence (B) is the correct answer.

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