---How did you work out the difficult chemistry problem?I worked it out by .A. working

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。