--- What has made him upset recently?
--- alone to face a troublesome case．
A. Left
B. Being left
C. Having left
D. To leave
网友回答
A. Left
B. Being left
C. Having left
D. To leave
解析：句意：--什么使他最近不安？--被单独留下面对棘手的情况。因为问句是what…?对主语提问，所以回答的是主语的内容，用动名词做主语，he和left是被动关系，用动名词的被动做主语，选B。