如图所示，草球甲、乙相互排斥，甲、丙相互吸引，如果已知甲带正电，那么乙、丙的带电情况是（ ）

   I think the best place to go on Sunday is the zoo. When you are 1, you can go there with your family.  2 the zoo, there are many animals: elephants, deer , 3, rabbits and other animals. Elephants are the 4 animals on land. I 5 there is a baby elephant in our zoo. So I want to 6 at it. Yu Xuan wants to go with me. She says, " 7 we go there together?" "Sure!" I say. "We can 8 a good time there. But let's 9 there at half past four. I have 10 homework to do."

 根据所学物质的性质及用途填空：

（1）用如图所示装置进行电解水实验，A试管中收集到的气体是 ____（填化学式），该实验说明水由 ____组成．

（2）用铁锅作炊具主要利用铁的 ____性，使用铁锅有利于人体健康，主要是因为适量补充铁元素，可预防或治疗 ____．

（3）写出符合要求的物质化学式：

①常用作建筑材料的碱 ____；

②人体胃液中含有的酸 ____；

③赤铁矿的主要成分 ____．