(2) As can be seen from the picture, the Three Gorges is located in the Yangtze River section between Yichang and Chongqing, so the tour group will pass through the Three Gorges the next day, followed by Qutang Gorge, Wuxia Gorge and Xiling Gorge from west to east;

(3) On the fourth and fifth days of the trip, some tourists experienced hypoxia such as shortness of breath, which was caused by altitude sickness, so the main reasons were high altitude and thin air;

(4) The five-day trip has passed through many terrain areas, namely, the southeast hills, the middle and lower reaches of the Yangtze River, the Sichuan Basin and the Qinghai-Tibet Plateau;

So the answer is: (1) Wushan; (2) 2; b； (3)B； (4) southeast; Middle and lower reaches of the Yangtze River; Sichuan.