题目

阅读下面短文，在空白处填入1个适当的单词或括号内单词的正确形式。John Snow got interested in two theories 【1】 (explain) how cholera killed people. He believed in the second theory that suggested that people absorbed the disease with their meals. In 1854，【2】 another outbreak hit London，he was ready to test the two theories. 【3】 the help of the map he made，he got a valuable clue about the cause of the disease and was able to announce that the water was to blame.下列有关书写化学方程式的原则或步骤中，叙述错误的是A.必须遵守质量守恒定律B.一定要注明反应条件C.要注明生成物的状态(气体、沉淀)D.在化学方程式配平的过程中，需要改动化学式的角码，这样可以使配平容易一些