What does the phrase “stepping stone

Câu hỏi: What does the phrase “stepping stone” in paragraph 4 mean?

A. An asset or possession prized as being the best of a group of similar things.

B. A person who travels without settling down for any significant period of time

C. An important clue to understanding something that is challenging or puzzling.

D. An action or event that helps one to make progress towards a specified goal.