The answer is C because in greek mythology, achilles was dumped into a river of some sorts(I don't specifically remember the name), and the only part of his body that did not go in was his heel due to his mom holding him by the heel. The river gave protection to the parts of the body that touched the river. thus, since his heel never entered, that was his one vulnerable spot on his body, intertwining with the theme mentioned in letter choice C.
