【题目】I just don’t understand _______ that prevents so many Americans from being as happy as one might expect.
A. why it does B. what it does
C. what it is D. why it is
在线课程【答案】C
【解析】
试题分析：分析句子结构可知understand后是强调句的特殊疑问句形式，“我就是不明白是什么阻止了这么多美国人过得快乐”，“what is it that prevents...”是强调句的特殊疑问句，作动词understand的宾语，要用陈述句语序，故选C。