“reduced the annual range of water temperature in the river below the dam from 50 degrees to 6 degrees”说的是temperature range的变化,而后面提到是“scientists have hypothesized that sharply rising water temperatures "说的是“rising water temperatures---正确答案A是对前者的例证,但是对rising temperrature没有作用啊,我觉得C选项对题目中的假设逻辑上更有针对性。