题目内容：
Questions are based on the following passage.
注意：此部分试题请在答题卡2上作答。
The problem that has been annoying cancer researchers for years is the difficulty in telling______________.
[A]how cancer cells are formed
[B]why cancer cells never stop dividing
[C]why normal cells can turn into cancer cells
[D]how different normal cells are from cancer cells
参考答案：
答案解析：