tokens: We thank the anonymous referee for significantly improving this paper by pointing out uncertainties that had been missed. This work is supported by the International Partnership Program of the Chinese Academy of Sciences No.114A11KYSB20160008, the National Natural Science Foundation of China No. 11373038, and the National Key Basic Research Program of China (973 Program) 2015CB857100, the China Ministry of Science. D.L. acknowledges support from the “CAS Interdisciplinary Innovation Team” program. This work was carried out in part at the Jet Propulsion Laboratory, which is operated for NASA by the California Institute of Technology. M.K. acknowledges the support of Special Funding for Advanced Users, budgeted and administrated by Center for Astronomical Mega-Science, Chinese Academy of Sciences (CAMS). N.M.M.G. acknowledges the support of the Australian Research Council through grant FT150100024. The Arecibo Observatory is operated by SRI International under a cooperative agreement with the National Science Foundation (AST-1100968), and in alliance with Ana G Méndez-Universidad Metropolitana, and the Universities Space Research Association. CO data were observed with the Delingha 13.7 m telescope of the Qinghai Station of Purple Mountain Observatory. We appreciate all the staff members of the Delingha observatory and Zhichen Pan for their help during the observations.
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