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Geographical locations of the study. Guangdong province is located in south China, and there are 21 cities, namely Guangzhou, Shenzhen, Foshan, Dongguan, Zhuhai, Zhongshan, Zhaoqing, Huizhou, Jiangmen, Chaozhou, Shantou, Jieyang, Meizhou, Shanwei, Shaoguan, Heyuan, Qingyuan, Yunfu, Yangjiang, Maoming and Zhanjiang, included in this study. The Pearl River Delta regions were marked with pink, and the Non-Pearl River Delta regions were indicated by light blue.
