题目内容：
What does "the million-dollar question" (Line 1, Para. 8) most suitably refer to?
A．It is a question that is worth a million dollars． B．It is a difficult question that no one can give a final answer to．
C．To answer the question will cost a huge amount of money．
D．There is too much research that is related to this question．
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答案解析：