A．knownB．knowledgeC．pillowD．yellow
A. known
B. knowledge
C. pillow
D. yellow
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阅读下面文字，完成下面的问题
在地球之外，究竟有没有外星人？两位美国学者撰文说，他们花了5年时间，在北半球天空捕捉到37个可能是来自地球外文明的讯号。
这两位学者是利用直径为26米的射电望远镜．来寻找由浩瀚宇宙深处发出的未知讯号的。他们以波长2l厘米和10 5厘米的电波对北半球天空的全部区域分别进行了三次和两次调查。许多从事探索地球外文明的科学家认为，波长为2l厘米的电波在宇宙空间中极为普通，如果地 球外文明要向其他星体发出讯号，就很有可能会使用这个波长。
文中的“三次和两次”是指()。
A. 向宇宙中的37个讯号分阶段进行调查的次数
B. 对北半球天空的全部区域进行分片调查的次数
C. 使用两种不同波长的电波分别进行调查的次数
D. 按照时间(天数)的长短分别进行调查的次数
How does the professor organize the discussion？
A. She cites research to support her arguments.
B. She gives a demonstration of her theory.
C. She debates the issues with her students.
D. She shares strategies that she developed.
Why does the professor say this:
A. To emphasize the point that he has just made
B. To indicate that another point will be made
C. To demonstrate that the point is his opinion
D. To regain the students' attention for the next point
According to Paragraph 3, what can be inferred about three-field system？
A. It increased productivity but was more labor intensive.
B. It made crops less vulnerable to weather conditions.
C. It spread from the Mediterranean to the rest of Europe.
D. It was suitable in regions that had richer soil.
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