The Attempt at a Solution
For this problem, my thinking was that since the top of the rod is fixed, then the only part that is elongating is the bottom, so I found the part elongating at AB and said that was the total elongation. I am not sure if this reasoning is sound. I suppose the other possibility would be to add up AB and BC to get the total, but then I don't know what the deformation at point B means. I am interpreting that as the elongation of BC.