用符号或化学式填空:(1)硫原子____,(2)显+2价的镁元素____,(3)8个水分子____.
一对双眼皮夫妇生了一个单眼皮的孩子,请分析回答:
Water is very dangerous sometimes. So we must be careful ______ it.
如图所示,三位旅行者从北京到上海,甲乘火车直达,乙乘飞机直达,丙先乘汽车到天津,再换乘轮船到上海,这三位旅行者中( )