若函数f（x）=a^x（0＜a≠1）在﹣1，2上的最大值为4，最小值为m，则m=____ 

改错题：写出题中错误的表述，并加以改正。
在学习东欧剧变和苏联解体的有关内容时，小张同学做了笔记。由于粗心，出现了几处错误，请聪明的你从下面的笔记中帮他找出来并改正。
从1987年下半年开始，东欧各国风云突变。由于各国执政党的改变，资本主义制度也随之发生改变。东欧剧变以后，南斯拉夫一分为三。苏联在赫鲁晓夫上台后，政治体制也发生急剧变化，国家权力分散。1990年底，苏联解体。      

C

    One of the easiest ways to keep fit is to do jogging. Jogging is the name for a very gentle(舒缓的) running-it is just a little faster than walking.

    Start slow joging 20 meters , then walk 20 meters. Little by little , if you are not feeling very tired , you may do more jogging and less walking . Finally , jog the whole way . It will be easy for you to start jogging for 15 minutes twice a week , slowly, increase this to 20—30 minutes every day .The longer you join in jogging , the more you enjoy it and the healthier you will become . Some people like jogging alone , and others enjoy doing it with friends . Jogging in groups makes more people keener(热心的)，because you just can not stay in bed while there is a group of friends waiting outside.

    If you want to start jogging , prepare a pair of comfortable shoes and a determined mind.

下列图中的a. b. c分别表示某捕食链中的第一、二、三营养级，每个营养级不同物种个体数量如A图所示（图中每一柱条代表一个物种）。经过一段时间后，各营养级生物的数量变化如B图所示。有关叙述正确的是（    ）