Argument C affirms that argument B (for the duty to say that Bob is away) is stronger than argument A (for the duty not to make that statement). Therefore, we can conclude that the defeat link from B to A is OUT (as a weaker argument cannot rebut a stronger one), while the defeat link from A to B remains IN. Therefore, B strictly defeats A (it defeats it without being defeated by it). Consequently, B is IN and A is OUT: we should tell the lie.
Obviously, the opposite conclusion would follow if we took a different view of priorities, such as the view that deontological arguments, warranted by generalizable rules, always have priority over consequentialist arguments.