汽车以额定功率从水平路面上坡时，司机换挡目的是（   ）

    One day a famous teacher was walking with his student. On the way, they saw a lake. They stopped and the teacher told the student, “I'm thirsty. Get me some water from that lake.”

    Just when the student reached the lake, a cow was walking in the lake. So the water became very dirty. He thought “I can't give the dirty water to my teacher!”

    He came back and told the teacher, “The water is very dirty. We can't drink it.” After about half an hour, the teacher asked the student to get some water again. So he went back to the lake.

    But the lake was still dirty. So he told the teacher the same thing. After some time, the teacher asked him to go back again. But this time, he found the water was clear. The mud in the water came to the bottom. So he got some water.

    The teacher looked at the water and said to him, “You did nothing to make the water clear. You just let it be… Your mind is also like that! When it is bothered, just let it be. Give it some time.”

19世纪80年代初，日本明治政府认识到“好事贪功，反而挫折人民自主独立之气势，养成百事依赖政府之风习”，“其弊害大不可测”，进而采取的措施是()

阅读材料，回答下列问题：

材料一  狂犬病是由狂犬病毒（Rabies  Virus）引起的人畜共患的传染病，发病后几乎全部死亡．早在1884年病毒发现之前，法国科学家巴斯德就发明了狂犬疫苗．

材料二  巴斯德将狂犬的唾液注射到兔的体内，兔患狂犬病而死，巴斯德将死兔脑和脊髓通过处理，弱化成疫苗，注射到正常狗的体内，狗获得了对该病的免疫力，后来有一男孩被狂犬咬伤，若不治疗，就会死亡，在狂犬病发病潜伏期内，巴斯德多次对其接种弱化病毒，男孩果然平安无事．

材料三   2015年9月6日，云南保山一个小女孩被恶狗咬伤，当日在诊所注射狂犬疫苗，但仍于10月2日不幸死亡．专家分析，对创伤深广、严重者，除按要求注射狂犬疫苗外，还应在24小时内注射高效抗病毒血清，才能有效达到防治狂犬病的目的．

曲线$y=4x-x^3$在点（－1，－3）处的切线方程是 (     )