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 geometryincentertrigonometry