b
$\mathrm{v}=\sqrt{\frac{\mathrm{T}}{\mu}}=\sqrt{\frac{\mathrm{Mg}_{\mathrm{eff}}}{\mu}}$
$=\sqrt{\frac{(1)(\mathrm{g}+\mathrm{a})}{\mu}}=\sqrt{\frac{12}{1.2 \times 10^{-3}}}$
$=100 \mathrm{\,m} / \mathrm{s}$
${\rm{t}} = \frac{l}{{\rm{v}}} = \frac{1}{{100}} = 0.01\,{\rm{s}}$