john snow found that the poiiuted water was—— for the outbre

答案是：A to blame
be blame for 对…应负责任
John Snow found that the polluted water was to blame for the outbreak of cholera
约翰斯诺慢慢发现受污染的水是此次霍乱爆发的罪魁祸首（应该对此次霍乱的爆发负责）.
再问： 不是应该是被动吗
再答： sb/sth be to blame for sth是一个固定短语，“某人/某事应为某事负责”的 意思，用主动的形式表达被动的含义，不能用被动语态。希望再次帮助到你！