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18. A constant direct current of uniform density j is flowing in an infinitely long cylindrical conductor.The conductor contains an infinitely long cylindrical cavity whose axis is parallel to that of the conductor and is at a distance l from it. What will be the magnetic induction $\overrightarrow{\mathrm{B}}$ at a point inside the cavity at a distance r from the centre of cavity? ​

$$\begin{gathered} \left(\mathrm{A}\right) \frac{{\mathrm{\mu }}_0\left(\mathrm{j}\times \mathrm{r}\right)}{2} \left(\mathrm{B}\right) \frac{{\mathrm{\mu }}_0\left(\mathrm{j}\times \mathrm{l}\right)}{2} \\ \left(\mathrm{C}\right) \frac{{\mathrm{\mu }}_0\left(\mathrm{j}\times \mathrm{l}\right)+{\mathrm{\mu }}_0\left(\mathrm{j}\times \mathrm{r}\right)}{2} \left(\mathrm{D}\right) \frac{{\mathrm{\mu }}_0\left(\mathrm{j}\times \mathrm{l}\right)-{\mathrm{\mu }}_0\left(\mathrm{j}\times \mathrm{r}\right)}{2} \end{gathered}$$