问题补充：
The plan for the new design occurred to Mr. Smithon his work recently.
A. while concentrated
B. while concentrating
C. while he concentrated
D. while concentrating himself
答案：
A. while concentrated
B. while concentrating
(答案→)C. while he concentrated
D. while concentrating himself
解析：C 此题易选B, 认为题干是个省略句, 过去分词作时间状语。如果是省略句的话，省略的成分应该与的主句的主语和be动词相同，而且主句的主语是the plan，显然是不能作过去分词concentrated的逻辑主语的。
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