A knowledge of history is ________ to the understanding of this article.
A. basic
B. fundamental
C. essential
D. elementary
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They deceived him into (sign) ________ the papers.
Please write a clear (define) ________ for the word "deep".
Coal mines are often ugly, and theri (develop) ________ has a serious effect on animal and plant life.
This is a new (discover) ________ in medical science.
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