阅读下面材料，在空白处填入适当的内容（1 个单词）或括号内单词的正确形式。
Tom and Jack are good friends. They have dreamed about _____( take) a great bike trip. It was Tom who first had the idea to cycle ______the entire Mekong River. They _____ (ride) bicycles to travel along the Mekong River, ______source is in Qinghai Province. When it enters Southeast Asia, it travels slowly through low valleys to the plains, _____ rice grows. ______ the difficulties , Tom and Jack were determined to begin their journey at an altitude of _____( much) than 5,000 metres in Qinghai Province. During their journey , they enjoyed the continuous change of the weather. _____(finally), they reached Dali in Yunnan Province , where David and Sam joined____.   ____ four of them  went on with their great bike trip.