Since the mid-1960's considerable research_____in embryo transplants in many countries.
问题描述：
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
答
Dsince..(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...