c
$\overrightarrow M = {\text{I}}\pi r_2^2\widehat {\text{k}} - {\text{I}}\pi {\text{r}}_1^2\widehat {\text{k}}$
$ = \pi {\text{I}}\left( {{{\text{r}}_2} + {{\text{r}}_1}} \right)\left( {{{\text{r}}_2} - {{\text{r}}_1}} \right)\widehat {\text{k}}$
$ = \frac{{22}}{7} \times 7 \times (0.3 + 0.2)(0.3 - 0.1)\widehat {\text{k}}$
$ = 22 \times 0.5 \times 0.1\widehat {\text{k}}$
$ = 1.1\widehat {\text{k}}\,{\text{A}} - {{\text{m}}^2}$