简述水的消毒常用方法。
参考答案：
氯消毒、紫外线消毒、臭氧杀菌。
The reading club in our school _____ three years ago and I have been one of the students _____ charge of it. [ ]
A. set up, on
B. was started, in
C. put up, at
D. created, in
如图所示，在我国古代护城河上都装有吊桥。若用l表示成对桥板的拉力F的力臂，在拉起吊桥的过程中，力臂l的变化以及Fl用的变化情况是
A.l始终在增加，Fl始终在增加
B.l始终在增加，Fl始终在减少
C.l先增后减，Fl始终在减少
D.l先减后增，Fl先减后增
