View of Dianchi Lake in Kunming, SW China's Yunnan

An aerial drone photo taken on Jan. 13, 2024 shows tourists enjoying the scene by the Dianchi Lake in Kunming, southwest China's Yunnan Province. The overall water quality of Dianchi Lake has been maintained for six consecutive years since it reached Class IV in the country's five-tier evaluation system for the first time in 2018. (Xinhua/Hu Chao)

This photo taken on Jan. 13, 2024 shows a view of the Dianchi Lake in Kunming, southwest China's Yunnan Province. The overall water quality of Dianchi Lake has been maintained for six consecutive years since it reached Class IV in the country's five-tier evaluation system for the first time in 2018. (Xinhua/Hu Chao)

An aerial drone photo taken on Jan. 13, 2024 shows tourists enjoying the scene by the Dianchi Lake in Kunming, southwest China's Yunnan Province. The overall water quality of Dianchi Lake has been maintained for six consecutive years since it reached Class IV in the country's five-tier evaluation system for the first time in 2018. (Xinhua/Hu Chao)

An aerial drone photo taken on Jan. 13, 2024 shows a view of the Dianchi Lake in Kunming, southwest China's Yunnan Province. The overall water quality of Dianchi Lake has been maintained for six consecutive years since it reached Class IV in the country's five-tier evaluation system for the first time in 2018. (Xinhua/Hu Chao)

This photo taken on Jan. 15, 2024 shows a view of the Dianchi Lake in Kunming, southwest China's Yunnan Province. The overall water quality of Dianchi Lake has been maintained for six consecutive years since it reached Class IV in the country's five-tier evaluation system for the first time in 2018. (Xinhua/Hu Chao)

An aerial drone photo taken on Jan. 16, 2024 shows staff members collecting litter in the Dianchi Lake in Kunming, southwest China's Yunnan Province. The overall water quality of Dianchi Lake has been maintained for six consecutive years since it reached Class IV in the country's five-tier evaluation system for the first time in 2018. (Xinhua/Hu Chao)

An aerial drone photo taken on Jan. 15, 2024 shows a view of the Dianchi Lake in Kunming, southwest China's Yunnan Province. The overall water quality of Dianchi Lake has been maintained for six consecutive years since it reached Class IV in the country's five-tier evaluation system for the first time in 2018. (Xinhua/Hu Chao)

Staff members collect litter in the Dianchi Lake in Kunming, southwest China's Yunnan Province, Jan. 16, 2024. The overall water quality of Dianchi Lake has been maintained for six consecutive years since it reached Class IV in the country's five-tier evaluation system for the first time in 2018. (Xinhua/Hu Chao)

This photo taken on Jan. 15, 2024 shows a view of the Dianchi Lake in Kunming, southwest China's Yunnan Province. The overall water quality of Dianchi Lake has been maintained for six consecutive years since it reached Class IV in the country's five-tier evaluation system for the first time in 2018. (Xinhua/Hu Chao)