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tokens: The authors thank the anonymous referee for comments and suggestions that improved the paper. The authors also thank J. Martin-Pintado for allowing us to utilize his 2001 7 mm observations of Sgr B2 North. C.D.P. and M.M.M.L. were supported by NSF grant AST-1211460. M.M.M.L. was additionally supported by NSF grant AST-1109395 and the Humboldt Foundation. T.P. acknowledges financial support through a Forschungskredit of the University of Zürich, grant No. FK-13-112, and from the DFG priority program 1537 “Physics of the interstellar medium.” R.S.K. acknowledges support from the Deutsche Forschungsgemeinschaft (DFG) via the SFB 881 The Milky Way System (sub-projects B1, B2, B5, and B8), and the SPP (priority program) 1573, Physics of the ISM. He also thanks the European Research Council for significant funding under the European Community’s Seventh Framework Programme (FP7/2007-2013) via the ERC Advanced Grant STARLIGHT (project number 339177). R.G.M. acknowledges support from program UNAM-DGAPA-PAPIIT IA101715.