In most parts of the world, many students help their schools make less pollution. They join “environment clubs”. In an environment club, people work together to make our environment clean.

    Here are some things students often do.

    No-garbage lunches. How much do you throw away after lunch? Environment clubs ask students to bring their lunches in bags that can be used again. Every week they will choose the classes that make the least garbage and report them to the whole school.

    No-car day. On a no-car day, nobody comes to school in a car. Not the students and not the teachers! Cars give pollution to our air, so remember: walk, jump, bike and run. Use your legs! It's lots of fun.

    Turn off the water! Did you know that some toilets can waste twenty to forty cubic (立方) meters of water an hour? In a year, it would fill a small river! In environment clubs, students mend those broken toilets. We love our environment. Let's work together to make it clean.

已知圆$C_1:(x-2{)}^2+(y-3{)}^2=1$,圆$C_2:(x-3{)}^2+(y-4{)}^2=9$,M,N分别是圆$C_1{C}_2$上的动点,P为x轴上的动点,则$\left|PM\right|+\left|PN\right|$的最小值为（　　）

已知f（x）是定义在R上且以4为周期的奇函数，当x∈（0，2）时，f（x）=ln（x²﹣x+b），若函数f（x）在区间﹣2，2上的零点个数为5，则实数b的取值范围是____ 

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