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
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C解析考查时态。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。