You are given three iron rods. Two of the rods are magnets but the third one is not. How could you use the three rods to definitively identify the unmagnetized rod?

a. Test the rods a pair at a time in all possible orientations to determine which two rods have the strongest force between them; the leftover rod is unmagnetized.
b. Find one pair of rods that attract when placed end‑to‑end, regardless of orientation; the leftover rod is unmagnetized.
c. Test different pairs of rods in all possible orientations to find a rod that is not attracted to either of the other two rods; that rod is unmagnetized.
d. Test the rods one pair at a time in each possible end‑to‑end orientation until finding a pair that repels; the leftover rod is unmagnetized.