human: Given a node-centered hypergraph: <hypergraph>, where nodes represent papers and hyperedges represent sets of papers co-cited by a source paper.

Details of the hypergraph:
Center node: "new research trends in unconventional oil and gas environmental issue a bibliometric analysis"
  Abstract: With the booming of unconventional gas production in the world, how to balance environment pollution risk and economy of unconventional gas have become a common dilemma around the world. The aim of this study is to elucidate the research about environmental issue brought with development of unconventional oil and gas industry. To achieve this goal, we present a bibliometrics overview of this field from 1990 to 2018. Firstly, this study outlines a basic statistical analysis over journals, publications, authors, institutions and documents. Secondly, VOSviewer is employed to visualize the collaborative relationship to show the link between different author, institutions, regions and journals. Finally, document bibliographic coupling, cooccurrence and keyword burst detection are analyzed to reveal the emerging trend and hot topic. The results indicate that among all countries, America was the most productive country as well as cooperated the most with other countries, followed by China, while the China University of Petroleum is the most productive institution in the world, with 105 publications. Additionally, most articles were classified as energy fuels, environmental sciences and geosciences multidisciplinary. Furthermore, based on emerging trends analysis, it was concluded that hydraulic fracturing technology has become a hot topic, other popular research topics include: energy policy and regulation of unconventional gas development, greenhouse gas emissions, energy and water consumption of unconventional gas life cycle assessment.

Question: Please tell me which class the center node belongs to. The 40 classes are: cs.NA(Numerical Analysis), cs.MM(Multimedia), cs.LO(Logic in Computer Science), cs.CY(Computers and Society), cs.CR(Cryptography and Security), cs.DC(Distributed, Parallel, and Cluster Computing), cs.HC(Human-Computer Interaction), cs.CE(Computational Engineering, Finance, and Science), cs.NI(Networking and Internet Architecture), cs.CC(Computational Complexity), cs.AI(Artificial Intelligence), cs.MA(Multiagent Systems), cs.GL(General Literature), cs.NE(Neural and Evolutionary Computing), cs.SC(Symbolic Computation), cs.AR(Hardware Architecture), cs.CV(Computer Vision and Pattern Recognition), cs.GR(Graphics), cs.ET(Emerging Technologies), cs.SY(Systems and Control), cs.CG(Computational Geometry), cs.OH(Other Computer Science), cs.PL(Programming Languages), cs.SE(Software Engineering), cs.LG(Machine Learning), cs.SD(Sound), cs.SI(Social and Information Networks), cs.RO(Robotics), cs.IT(Information Theory), cs.PF(Performance), cs.CL(Computation and Language), cs.IR(Information Retrieval), cs.MS(Mathematical Software), cs.FL(Formal Languages and Automata Theory), cs.DS(Data Structures and Algorithms), cs.OS(Operating Systems), cs.GT(Computer Science and Game Theory), cs.DB(Databases), cs.DL(Digital Libraries), cs.DM(Discrete Mathematics). Directly output the class name.

gpt: cs.DL(Digital Libraries)