PART TWO: LANGUAGE KNOWLEDGE （45分）
Section A （15分）
Directions: Beneath each of the following sentences there are 4 choices marked A, B, C and D. Choose one answer that best pletes the sentence.
21. Trees are useful to man mainly in three ways, the most important of which is that they can protect from droughts (干旱) and floods.
A. it B. itself C. himself D. Him
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