如图所示，用拇指与中指压一支铅笔的两端，____（选填：拇指、中指）受到的压强大，若拇指受到的压力为2N，笔尖的受力面积为0.2mm²，则中指受到的压强为____ Pa．

—What's in the e____?

—There's a piece of paper in it.

阅读短文，选择正确答案。
    We are students of Yuying Primary School(小学). In our school, there are many teachers and students. There are twenty­six girls and twenty­four boys in our class. Our classroom is big. There are six windows and two doors. The walls(墙) are white. There are two pictures on the wall. A big blackboard is in the front of the classroom. Our desks and chairs are new. Our classroom is very nice. I like it very much.

计算小数加减法时，先把各数的____对齐，再按照____进行计算．

下列各组混合物中，能用溶解、过滤、蒸发的方法进行分离的是