Aerial photo taken on Dec. 29, 2017 shows the Erlang Mountain tunnel entrance on Ya'an-Kangding express highway in southwest China's Sichuan Province. The Caoba to Luding section of Ya'an-Kangding express highway was integrated into the highway network and began its trail operation on Sunday, which means all the 21 cities or prefectures in Sichuan are now connected with express highway. (Xinhua/Jiang Hongjing)