A. However, the severed head could not grow back. If fire could be applied at once to the amputated part.
B. To get rid of this monstrosity was truly a Herculean task, for as soon as one head was cut off, two new ones replaced it.
C. Hercules accomplished the labour through the aid of an assistant who cauterised the necks as fast as Hercules cut off the heads!
D. One of the Twelve Labours of Hercules was the killing of Hydra, a water monster with nine heads.
(CAT 1996)
BA is the most logical link because A represents a loophole (Which is for Hercules' benefit) and C explains how he exploits this loophole. Thus the logical link in DBAC