By the middle of the 1920s the farthest corners of the earth        explored.

         A．are       B．were      C．have been          D．had been

By the middle of the 1920s the farthest corners of the earth        explored.

       A．are    B．were C．have been      D．had been

By the middle of the 1920s the farthest corners of the earth        explored.

       A．are      B．were     C．have been         D．had been

By the middle of the 1920s the farthest corners of the earth        explored.

A．are          B．were           C．have been           D．had been

By the middle of the 1920s the farthest corners of the earth       explored.

By the middle of the 1920s the farthest corners of the earth ________ explored．

1. A.
   are
2. B.
   were
3. C.
   have been
4. D.
   had been