Only when they were told of the importance of time ________ wasting time is like wasting part of their lives．

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

Only when they were told of the importance of time ________ wasting time is like wasting part of their lives．

A．those boys realized B．those boys had realized

C．had those boys realized   D．did those boys realize