A copper rod of 1 cm diameter and 8 cm length is drawn into a wire of uniform diameter and 18 m length. The radius (in cm) of the wire, is :

Answer: Option B

Volume of copper rod :
$$\begin{array}{rl}& =\left(\pi \times \frac{1}{2}\times \frac{1}{2}\times 8\right)\text{ c}{\text{m}}^3\\ & =2\pi \text{ c}{\text{m}}^3\end{array}$$
Let the radius of the wire be r cm
Then, volume of wire :
$$\begin{array}{rl}& =\left(\pi r^2\times 1800\right){\text{cm}}^3\\ & =1800\pi r^2{\text{ cm}}^3\\ & \therefore 1800\pi r^2=2\pi \\ & \Rightarrow r^2=\frac{2}{1800}\\ & \Rightarrow r^2=\frac{1}{900}\\ & \Rightarrow r=\sqrt{\frac{1}{900}}=\frac{1}{30}\end{array}$$