tokens: This work is supported by Technology under State Key Development Program for Basic Research (973 program) No. 2012CB821802 and National Key Basic Research Program of China (973 Program) 2015CB857100, the China Ministry of Science, and the Guizhou Scientific Collaboration Program (#20130421). We are grateful to the anonymous referee for his/her constructive suggestions, which have greatly improved this paper. The authors would like to thank Pei Zuo for helping Delingha CO observations, John Dickey for discussing H i self-absorption, Xiaohu Li for discussing chemical evolutionary model. We thank the Pineda team for providing C and CO data. Part of CO data were observed with the Delingha 13.7 m telescope of the Qinghai Station of Purple Mountain Observatory. We appreciate the help of all the staff members of the observatory during the observations.
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