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(D) According to Ampere's circuital law,the magnetic field $B$ at a distance $r$ from the axis of a current-carrying conductor is given by $\oint B \c

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$P$ but not at $Q$ and $R$.

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(D) According to Ampere's circuital law,the magnetic field $B$ at a distance $r$ from the axis of a current-carrying conductor is given by $\oint B \cdot dl = \mu_0 I_{enclosed}$.For point $P$,which is in the hollow cavity,the enclosed current $I_{enclosed} = 0$,so the magnetic field $B_P = 0$.For point $Q$,which is inside the material of the conducting pipe,the enclosed current is non-zero,so the magnetic field $B_Q 
eq 0$.For point $R$,which is outside the pipe,the total current enclosed by an Amperian loop passing through $R$ is the total current $I$ flowing through the pipe. However,for a long straight cylindrical conductor,the magnetic field outside is $B = \frac{\mu_0 I}{2 \pi r}$. This is only zero if $I=0$ or $r 	o \infty$. Thus,$B_R 
eq 0$.Therefore,the magnetic field is zero only at point $P$.

Current is flowing through a conducting hollow pipe whose cross-section is shown in the figure. The value of magnetic induction will be zero at:

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