【正确答案】 1、did people realize how serious the pollution was    

【答案解析】[考点] ①倒装结构[考频：1]；②宾语从句[考频：1]
[解析] Not until引导的时间状语前置时，引起部分倒装，如果Not until引导的时间状语不前置，则不用倒装，所以本句可以改写为People did not realize how serious the pollution was until all the fish died in this river．