问题补充：
单选题SincetheObamaadministrationtookoffice,theChina—USrelations________positivedevelopmentthankstojointeffortsofbothsides,PresidentHusaidattheNuclearSecuritySummitinWashington,April,12,.A.witnessedB.were witnessedC.had witnessedD.have witnessed
答案：
D
