问题补充：
halfway through the exercise, the teacher changed his mind and collected all our papers.
A. When only being
B. When we were only
C. Being only
D. Having been
答案：
A. When only being
(答案→)B. When we were only
C. Being only
D. Having been
解析：我们只做了一半练习老师就改变了主意把我们的试卷收了上去。因为主句与从句的主语不一致所以不能用分词做状语故选B
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