It was not until I           your letter that           the true state of affairs.

A．have read; understand                                  B．read; had understood

C．had read; would understand                         D．had read; understood

It was not until I ________ your letter that I ________ the true state of affairs．

1. A.
   have read; understand
2. B.
   read; had understood
3. C.
   had read; understood
4. D.
   had read; would understand