Only when he returned from the exchange program ________ how much he enjoyed it.

A．he realized　　 B．he had realized　

C．did he realize　　 D．had he realized

Only when he returned from the exchange program ________ how much he enjoyed it．

1. A.
   he realized
2. B.
   he had realized
3. C.
   did he realize
4. D.
   had he realized