Since the mid-1960's considerable research_____in embryo tra
Since the mid-1960's considerable research_____in embryo transplants in many countries. A has carried out B was carried out C carried out D has been carried out 这个题的谓语是哪个呀? 不对,是这题的主语是哪个啊
D since..(the mid-1960's) 用于完成时的时间状语,表明本句用完成时 主语是considerable research,大量的研究,research研究是抽象名词不可数,视为单数概念,因此要用has,还有研究不是自己做的,被做的,因此要用被动,就是has been carried out啊