Whathaspuzzledthescientistsisthataslightchangeinitsenvi...

问题详情：

What has puzzled the scientists is that a slight change in its environment ______ have pushed the rare species over the edge into extinction.

       A. would                      B. must                        C. shall                         D. should

【回答】

D

知识点：情态动词

题型：选择题