People enjoy boating on the Old Danube in Vienna, Austria, on July 31, 2020. The Old Danube, a centrally located recreational area in Vienna, is popular among Viennese in summer. (Xinhua/Guo Chen)

People enjoy boating on the Old Danube in Vienna, Austria, on July 31, 2020. The Old Danube, a centrally located recreational area in Vienna, is popular among Viennese in summer. (Xinhua/Guo Chen)

People enjoy boating on the Old Danube in Vienna, Austria, on July 31, 2020. The Old Danube, a centrally located recreational area in Vienna, is popular among Viennese in summer. (Xinhua/Guo Chen)

People enjoy boating on the Old Danube in Vienna, Austria, on July 31, 2020. The Old Danube, a centrally located recreational area in Vienna, is popular among Viennese in summer. (Xinhua/Guo Chen)

People enjoy boating on the Old Danube in Vienna, Austria, on July 31, 2020. The Old Danube, a centrally located recreational area in Vienna, is popular among Viennese in summer. (Xinhua/Guo Chen)

People enjoy boating on the Old Danube in Vienna, Austria, on July 31, 2020. The Old Danube, a centrally located recreational area in Vienna, is popular among Viennese in summer. (Xinhua/Guo Chen)