①小赵、小钱、小孙、小李到4个景点旅游，每人只去一个景点，设事件A=“4个人去的景点不相同”，事件B=“小赵独自去一个景点”，则 $P=(A|B)=\frac{2}{9}$ ；

②设函数f（x）存在导数且满足 $\underset{△x\to \infty }{lim}\frac{f\left(2\right)-f\left(2-3△x\right)}{3△x}=-1$ ，则曲线y=f（x）在点（2，f（2））处的切线斜率为﹣1；

③设随机变量ξ服从正态分布N（μ，7），若P（ξ＜2）=P（ξ＞4），则μ与Dξ的值分别为μ=3，Dξ=7．

下列结构中均能让某些大分子物质直接通过的一组是(   )

根据短文内容，选择正确答案。

      Our living environment is being polluted faster than before and it seems nobody can prevent it. Time brings us more people, and more people bring us more industry, more cars, larger cities and growing use of man-made materials.

      How can we understand and solve this problem? The fact is that pollution is caused by man —by his desire (欲望）for a modem way of life. So we are often ready to offer everything: clean water, good food, our health and the future of our children. There are more people from the countryside moving into the cities and enjoying the modem society. But as the technological momentum (势头) has grown in the last twenty years, pollution has become a serious problem.

      Now we have to stop to ask ourselves where we are going and why. It makes one think of the story about the airline pilot who told his passengers over the loudspeaker, “I've got some good news and bad news. The good news is that we're going at 530 miles per hour. The bad news is that we're lost and don't know where we are going.” The sad fact is that this problem becomes a true story when speaking of our modem society.