They studied what since 2007 and found Australian electricity prices were among the highest in the developed world.
A. has happened
B. happened
C. had happened
D. has been happening
网友回答
A. has happened
B. happened
(答案→)C. had happened
D. has been happening
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。