b
(b)\(i = \frac{q}{t} = 100 \times e\) \({B_{{\rm{centre}}}} = \frac{{{\mu _o}}}{{4\pi }}.\frac{{2\pi i}}{r} = \frac{{{\mu _o}}}{{4\pi }}.\frac{{2\pi \times 100e}}{r}\) \( = \frac{{{\mu _o} \times 200 \times 1.6 \times {{10}^{ - 19}}}}{{4 \times 0.8}} = {10^{ - 17}}{\mu _o}\)