This combination photo shows contestants taking part in the traditional Tray Race in front of the Ruins of St. Paul's complex in Macao, south China on Sept. 27, 1999 (left, photo by Wang Yuguo) and on Sept. 27, 2019 (right, photo by Cheong Kam Ka), respectively. On Dec. 20, Macao will celebrate the 20th anniversary of its return to motherland. Over the past two decades, the special administrative region has made great strides in economic development and achieved prosperity and stability under the "one country, two systems" principle. Since Dec. 20, 1999, the historic day for Macao, what has changed? And what remains unchanged? Our selection of photos present comparative illustrations of the south China city's past and present. (Xinhua)