The bob of a pendulum of mass $m$ and length $L$ is displaced, $90^{\circ}$ from the vertical and gently released. In order that the string may not break upon passing through the lowest point, its minimum strength must be
(a) $m g$
(b) $2 \mathrm{mg}_{\mathrm{J}}$
(c) $3 \mathrm{mg}$
(d) $4 \mathrm{mg}$