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They studied what ______ since 2007 and found Australian electricity prices were among the  highest in the developed world.
A.has happenedB.happenedC.had happenedD.has been happening
C
考查时态。根据studied知道句子本身是过去时,2007年发生的事情正是研究的对象,说明他们是在研究之前发生的,所以应该使用过去完成时。
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