根据材料，回答第 31～35 题：

第 31 题 What does the author mean by "floats" ( Line 2, Para.1 ) ?（　　）

第 48 题

第 46 题

第 47 题

第 45 题 The offspring of the supertrees have proved to be genetically superior to those of the average trees.

第 44 题 Some scientists have the suspicion that the surviving forests have lost much of their genetic quality be-cause they were the offspring of poor specimens.

根据材料，回答第 41～45 题：

第 41 题 Water in the Yellowstone River turned dark brown because of the spring rain.

第 43 题 In the days when Europeans first came to America, it had one of the largest primeval forests in the world.

第 42 题 The cottonwood tree Mr. Flanagan found was an extremely tall tree with broad leaves.

第 40 题 According to the text, Tursich __（　　）

第 39 题 It is risky to invest in alternative energy because _______ .（　　）