It was when searching for the lost book whose cover was missing ______ a secret letter addressed to his mother.

It was when searching for the lost book whose cover was missing ______ a secret letter addressed to his mother.

1. A.
   what he found
2. B.
   until he found
3. C.
   that he found
4. D.
   he had found

It was when searching for the lost book whose cover was missing        a secret letter addressed to his mother．

    A．did he find    B．until he found  C．that he found D．had he found

It was when searching for the lost book whose cover was missing         a secret letter     addressed to his mother．

A．did he find        B．until he found           C．that he found          D．had he found

It was when searching for the lost book whose cover was missing ______ a secret letter addressed to his mother.