问题
选择题
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
答案
答案：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。