PART TWO: LANGUAGE KNOWLEDGE 
Section A 
Directions: Beneath each of the following sentences there are 4 choices marked A, B, C and D. Choose one answer that best completes 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