These achievements_______ without the contribution of Qian Xuesen, a key leader of the New China's nuclear, missile and so on．A.would have been unthinkableB.would not be unthinkableC.would be unthinkableD.would not have been unthinkable
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A解析（本题考查虚拟语气。题干句子中“without the contribution of Qian Xuesen”表示与过去事实相反的虚拟语气，谓语动词要用“would（not）}lave done'’。）