Mountain and river migrants

Test analysis: The information of "Shaanxi-Gansu-Ningxia Border Region" appeared in the materials. The Shaanxi-Gansu-Ningxia Border Region is the proper name of the northern Shaanxi base area where the Central Committee of China is located after the outbreak of the Japanese full-scale war of aggression against China and the second cooperation between the two countries. The combination option should appear in the background of cooperation among countries after the August 13th Incident in 1937, so C is chosen. A is 193 1 years ago, b is 1936 years ago and d is 1945 years later.