21. The 14N and 15N derivatives of TpOs(NS)Cl2 [Tp = hydrotris(1-pyrazolyl)borate, a tridentate ligand] have been prepared. The 14N derivative has a nitrogen-sulfur stretch at 1284 cm-1. Predict the N-S stretch for the 15N derivative. (A) 1314 cm-1 (B) 1254 cm-1 (C) 1224 cm-1 (D) 1194 cm-1