Let I be the incenter of △ABC, and O be the excenter corresponding to B. If ∣BI∣=12, ∣IO∣=18, and ∣BC∣=15, then what is ∣AB∣?<spanclass=′latex−bold′>(A)</span>16<spanclass=′latex−bold′>(B)</span>18<spanclass=′latex−bold′>(C)</span>20<spanclass=′latex−bold′>(D)</span>22<spanclass=′latex−bold′>(E)</span>24