2 . When water is cold, people will find it uncomfortable to swim. Wetsuits(潜水服）can help keep people warm.    1    So wetsuits are important. In many water sports, you can find people wearing them.

The wetsuit doesn't have a long history. According to studies, the wetsuit only began during WWII when Italians started wearing wetsuits that were made of rubber. And it was widely believed in 1951, Hugh Bradner began working to improve the equipment.     2    

Now there are many kinds of wetsuits. Contrary to what some may think, wetsuits do not work by keeping the diver(潜水员）dry; they are designed to let water into the suit. This thin layer of water is trapped（保存）between the diver and the suit. And it's allowed to move between the skin of the diver and the inside layer.     3    And it can help keep the diver comfortable during the diving process. That's why it's important to have a nice, tight fit for a wetsuit.

    4    The shorter wetsuit often provides cover for the main part of people's bodies. This kind of wetsuit works well with boating, water skiing, swimming, etc. A longer wetsuit can completely cover the body, including long legs.With more protection for the body, it may be used in cool weather conditions.

Wetsuits can also differ in thickness. They range from about 0. 5mm to 8mm in thickness. It's true that thicker wetsuits are more likely to affect your movement.     5    Generally, the colder the water you plan to dive in, the thicker the wetsuit should be.

4 . When I was a practice teacher in a middle school，the students in my class were always making ______，and had no interest in chemistry. So I decided to do ______to change their______ towards chemistry.

The evening before I would teach all by myself，I gave a piece of paper to everyone. I told them to write ______ they wanted on the paper and handed it in and then I could tell what each one had ______ .All the students said I _____ be longsighted(远视)．Then I stood ______ my back towards them. I didn't turn back ______ they had finished and ______ all the paper in a cardboard case.

I turned the cardboard case with the bottom(底部)towards the students，and told them calmly what each one had written on the paper. The students were surprised，______ that I had learned their ______ completely in such a short time.

I told them the truth and they were ______ at my brightness：there were five lines of students in the classroom，and every line had ten students. I had written serial number(from 1 to 50 )on every ______ of the paper in a chemical reagent(试剂)that cannot be seen ______eyes，and then gave ______out to the students according to the groups of their seats. After the paper was ______ in，I used______ chemical reagent onto the corner of the paper. Thus the serial number was ______！

______ ，the students became ______ interested in chemistry，and we became good friends.