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,12,2010.

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,12,2010.

1. A.
   witnessed
2. B.
   were witnessed
3. C.
   had witnessed
4. D.
   have witnessed

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,12,2010.

A．witnessed      B．were witnessed     C．had witnessed            D．have witnessed

 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,12,2010.

A．witnessed       B．were witnessed     C．had witnessed        D．have witnessed