Guide Gong Zhaoyi explains the models of China's first atomic bomb (R) and hydrogen bomb at an exhibition hall in the Liangdancheng, meaning the city of nuclear bombs, the site where China's first atomic and hydrogen bombs were designed, in Zitong County of Mianyang, southwest China's Sichuan Province, April 23, 2019. Zitong was home to the research headquarters of China's nuclear weapons program, where a dozen of world-class scientists worked for over a decade on China's first atomic and hydrogen bombs and satellite launches in the 1960s and 1970s. (Xinhua/Liu Kun)