Since the Obama administration took office the China—US relations positive development t

问题补充：

Since the Obama administration took office, the China—US relations positive development thanks to joint efforts of both sides, President Hu said at the Nuclear Security Summit in Washington, April,12,.

A. witnessed

B. were witnessed

C. had witnessed

D. have witnessed

答案：

A. witnessed

B. were witnessed

C. had witnessed

(答案→)D. have witnessed

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