题目内容：
Directions：In this section,there is a short passage with 5 questions or incomplete statements．Read the passage carefully．Then answer the questions or complete the statements in the fewest possible words on Answer Sheet2． Questions are based on the following passage．
注意：此部分试题请在答题卡2上作答。
At the beginning of the passage，the writer points out that not enough is known about________________________________.
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答案解析：