Two trains $A$ and $B$ each of length $400 \mathrm{~m}$ are moving on two parallel tracks with a uniform speed $72 \mathrm{~km} \mathrm{~h}^{-1}$ in the same direction with $A$ ahead of $B$. The driver of $B$ decides to overtake $A$ and accelerates by $1 \mathrm{~m} \mathrm{~s}^{-2}$. If after $50 \mathrm{~s}$, the guard of $B$ just brushes past $A$, what was the original distance between them?
(a) $750 \mathrm{~m}$
(b) $1000 \mathrm{~m}$
(c) $1250 \mathrm{~m}$
(d) $2250 \mathrm{~m}$