They _ (investigate) the causes of gas explosion since last week.这里怎么填?

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They _ (investigate) the causes of gas explosion since last week.这里怎么填?
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They _ (investigate) the causes of gas explosion since last week.这里怎么填?
They _ (investigate) the causes of gas explosion since last week.这里怎么填?

They _ (investigate) the causes of gas explosion since last week.这里怎么填?
填写:have investigated
解析:since的用法:译作“自从…以来”;接时间点:如 I have stayed in that country since 1995.要求:谓语动词.用现在完成时,而且须是延续性动词.
翻译:他们从上个周开始就一直在调查瓦斯爆炸的起因