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
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答案解析: