b
$\overrightarrow{ M }=- i \pi(0.5)^{2} \hat{ k }+ i \pi(0.3)^{2} \hat{ k }$
$\overrightarrow{ M }=-7 \times \frac{22}{7}\left(\frac{25}{100}-\frac{9}{100}\right) \hat{ k }$
$=-22\left(\frac{16}{100}\right) \hat{ k }$
$\overrightarrow{ M }=-3.52 \hat{ k }\,Am ^{2}$
$=-\frac{7}{2} \hat{ k }\,Am ^{2}$
