答案：选A。此处believed to have occured due to the burning of construction materials 为过去分词短语做非限制性定语，对先行词fire加以补充说明，可转化为非限制性定语从句which was believed to have occured due to the burning of construction materials 。此处使用完成式表示动词【occur】的动作发生在believe之前。若用B则强调在其后发生。
due to一般用在be动词之后做表语，但实际运用中也可以用来作状语。
如：1）She has been absent from work due to illness.
2）The workforce was reduced, partly due to budget pressures.
3）He arrived late due to the storm．他由于暴风雨而来迟了。
时态不同，A是完成时态，B是普通的不定式，应该选A吧
b
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