Not a cool answer. How could C be seised? B has a free tenement that precedes that of C. Just because C a future interest in a fee simple absolute does not mean that C will be seised. Seisin speaks of lawful possession of a free tenement. That will not be C. When B dies, since B only had an estate for B's own life, that estate will disappear. At that point, but not before, C or, if C happens to be dead, C's heirs or assigns will then be seised for the duration of the leasehold, although A will continue to hold the lease and be on the land, and be able to keep C, C's heirs, or C's assigns off. Seisin is not the same as possession or protectability.