Why did the mercenaries not work out for the romans?

For Sidonia Apollinaire and his beloved native city of Clermont, the year 471 could hardly have brought more misery. Goths surrounded the proud Roman city. Morale was low. Defeat seemed inevitable. Then, quite literally, the cavalry arrived. A small group of men—just nineteen—charged across the plain to scatter the Gothic host. The sheer audacity of the surprise attack must have stunned the besieging army. The Goths, who are said to have numbered in the thousands, suffered heavy losses, and the city was delivered.

The answer is b