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）
小题1:As you go up a mountain, you have to overcome the earth’s g           which pulls you down.
小题2:The m             leaf has become the most important symbol of Canada since the national flag was introduced in 1965.
小题3:The little girl was t          at the thought of being left alone at home and cried.
小题4:He went to the United States in the 1980s and s                down there.
小题5:Don’t leave matches or cigarettes on the table w              reach of little children.
小题6:How quickly a fuel burns depends on how well it is mixed with              (氧气) or air.
小题7:The policemen were sent to find the black box because it showed why the plane had           
(坠毁).
小题8:The idea                 (存在) only in the minds of us young people today.
小题9:This problem is only                 (稍微) difficult so I can work out.
小题10:They claimed that when they were               (包围) by the gang they acted in self-defense.