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
22. We know a good deal about the people who lived in China 4, 000 years ago, because they ______write and leave written records for those who lived after them.
A. could B. should C. must D. shall
