 Complete the following phrase:  In October 1939, he was assigned as a junior flight division leader junior buntaichō to the 12th Air Group, which was stationed in Central China. His unit flew Aichi D1A dive bombers out of the bases in Anqing, Jiujiang, and Hankou to support ground operations in the area. In May 1940, the unit switched to the new Aichi D3A dive bombers and participated in the capture of Yichang. Lieutet Arima frequently attacked the supply shipping on Yangtze west of Yichang, which was particularity challenging for dive bombing as the Gorges can have more than 1000 meters cliffs on both sides. In September, he started to fly missions against 