___come out of the building,my first thought was to call the fire department.

答案：C
翻译：当我看见烟从房子里冒出来时,我的第一念头就是给消防局打电话.
1.AD都使用了非谓语结构做状语,这就要求非谓语动词see的逻辑主语必须是句子的主语my first thought,这样明显不符合逻辑,因为逻辑主语明显应该是I,而不是my first thought.
2.如果选B,则要求是：As soon as the smoke was seen (to) come out of the building
不带to的不定式做宾语补足语时,变成被动语态后要补上to,由此排除B.
3.综上所述,只有答案C为正解.
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