问题补充：
---How did you work out the difficult chemistry problem?
--- I worked it out by .
A. working with a group
B. think very hard
C. ask the teacher for help
D. doing oneself
答案：
(答案→)A. working with a group
B. think very hard
C. ask the teacher for help
D. doing oneself
解析：句意：你是如何解决这个化学难题的？下文by是介词，表示通过某种方式，后面接名词或者动名词作宾语，选项A表示通过小组合作，选项D语义不符，结合语境可知选A。
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