—How unhappy your roommate looks!
—Yes, but he's not willing to tell me what it is is troubling him.
A. what
B. which
C. as
D. that
网友回答
A. what
B. which
C. as
(答案→)D. that
解析：选D 考查强调句型。分析句子结构可知，此处为特殊疑问句的强调句式，其结构为“特殊疑问词＋is＋it＋that＋从句”，又因强调句在句中作tell的宾语，应用陈述句语序。故选D。