根据句意及首字母或汉语提示，写出各句中所缺单词的正确形式。（每空一词，共10分）
As you go up a mountain, you have to overcome the earth’s g           which pulls you down.
The m             leaf has become the most important symbol of Canada since the national flag was introduced in 1965.
The little girl was t          at the thought of being left alone at home and cried.
He went to the United States in the 1980s and s                down there.
Don’t leave matches or cigarettes on the table w              reach of little children.
How quickly a fuel burns depends on how well it is mixed with              (氧气) or air.
The policemen were sent to find the black box because it showed why the plane had           
(坠毁).
The idea                 (存在) only in the minds of us young people today.
This problem is only                 (稍微) difficult so I can work out.
They claimed that when they were               (包围) by the gang they acted in self-defense.

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