题目内容
13．Please knock on the door beforeenteringthe teachers'office．（enter）分析 请在进入老师办公室之前敲门．
解答 答案：entering．
before意思是"在…之前"，是介词，后面跟动词的话，要用动词的ing形式，结合给出的英语提示enter，所以要用entering．
点评 首先要掌握在介词后面需要用动词的ing形式，然后结合具体的题目，就可以确定正确答案．
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3．March 20th is World Water Day every year．According to a report from the United Nations，more than half of the world's major rivers have serious water loss and are polluted．
The Yellow River，the world's fifth longest river，watered some of the oldest farms on earth．（46）D Its source（源头）on Qinghai-Tibet Plateau is drying up．In most of the past 35years，it didn't reach the sea．
The Nile in Egypt，running 6，650km，is the longest river in the world．（47）C Now it is down to 2billion m3or so．Australia's Murray River reaches the sea only every other year．One fifth of America's Rio Grande has no water，either．
（48）B
Mostly it's damming（筑坝）and global warming that make rivers dry．（49）A Now about 45，000of them lie across the world's rivers．They hold back around 15% of the world's fresh water from running to the sea．
（50）E Warmer weather takes water away．The glacier（冰川）are becoming smaller．The source of the Yellow River is drying up because of it．Global warming has had certain effects on water sources．Droughts （干旱） happening around the world make people pay more attention to this fact．
The Yellow River，the world's fifth longest river，watered some of the oldest farms on earth．（46）D Its source（源头）on Qinghai-Tibet Plateau is drying up．In most of the past 35years，it didn't reach the sea．
The Nile in Egypt，running 6，650km，is the longest river in the world．（47）C Now it is down to 2billion m3or so．Australia's Murray River reaches the sea only every other year．One fifth of America's Rio Grande has no water，either．
（48）B
Mostly it's damming（筑坝）and global warming that make rivers dry．（49）A Now about 45，000of them lie across the world's rivers．They hold back around 15% of the world's fresh water from running to the sea．
（50）E Warmer weather takes water away．The glacier（冰川）are becoming smaller．The source of the Yellow River is drying up because of it．Global warming has had certain effects on water sources．Droughts （干旱） happening around the world make people pay more attention to this fact．
| A．Dams provide fields with water and make power． |
| B．How does it happen？ |
| C．It used to carry 32billion m3of water a year． |
| D．But now it has a serious problem． |
| E．Global warming is making things worse． |