There are several research centers in China ________ a certain disease called Bird Flu is being studied.

A. which       B. where    C. when     D. what

There are several research centers in China ________ a certain disease called Bird Flu is 
being studied. 

There are several research centers in China ________ a certain disease called Bird Flu is 

being studied. 

A．which            B．where            C．when             D．what

There are several research centers in China ________ a certain disease called Bird Flu is being studied.

1. A.
   which
2. B.
   where
3. C.
   when
4. D.
   what