A bucket tied to a string is lowered at a constant acceleration of $\frac{g}{4}$. If the mass of the bucket is $m$ and is lowered by a distance $d$, the work done by the string will be:
(a) $\frac{m g d}{4}$
(b) $-\frac{3}{4} m g d$
(c) $-\frac{4}{3} m g d$
(d) $\frac{4}{3} m g d$