若指数函数$y=a^x$在－1,1上的最大值与最小值的差是1，则底数a等于(  )

修改下面的病句。

①爬山虎，牵牛花，紫藤等是可供垂直绿化的植物之一。

②雅鲁藏布大峡谷可以堪称世界第一的壮丽景观。

③《题西林壁》的作者苏轼生活的时代是四川眉山。

    We are learning English, but how can we learn English well? A student can know a lot about English, but maybe he can not speak English.
If you want to know how to swim, you must get into the river. And if you want to be a football player, you must play football. So , you see, you can learn English only by using it. You listen to your teacher carefully in class. You must read your lessons every day. You must speak English to your English teacher and classmates and also you must write in English. Then one day, you may find your English very good.

对下列两个句子的理解，正确的一项是(    )
(1)希望是本无所谓有，无所谓无的。这正如地上的路；其实地上本没有路，走的人多了，也便成了路。   一鲁迅《故乡》
(2)其实地上本来就有路，走的人多了，也就没有了路。——重庆洋人街标语

定义映射$f:A\to B$，若集合A中元素在对应法则f作用下象为${\log }_{3}x$，则A中元素9的象是(      )