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
考查時(shí)態(tài)。根據(jù)studied知道句子本身是過(guò)去時(shí)，2007年發(fā)生的事情正是研究的對(duì)象，說(shuō)明他們是在研究之前發(fā)生的，所以應(yīng)該使用過(guò)去完成時(shí)。
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