题目：
选择题
Advertising first became important in the last 15th century, ＿＿＿＿＿＿＿＿ the merchants of the rapidly growing cities and towns needed a way to tell people ＿＿＿＿＿＿ their goods could be bought.
- A. then, where
- B. when, how
- C. then, how
- D. when, where
答案：
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