CCTV is exactly like a window on the world ________ you will sit by it and fix your attention on what it shows.

1. A.
   if
2. B.
   as
3. C.
   while
4. D.
   unless

CCTV is exactly like a window on the world ____you will sit by it and fix your attention on what it shows .

  A if  B as  C while  D unless

CCTV is exactly like a window on the world ____you will sit by it and fix your attention on what it shows .

  A if  B as  C while  D unless

CCTV is exactly like a window on the world ________ you will sit by it and fix your attention on what it shows.

[　　]

CCTV is exactly like a window on the world ________ you will sit by it and fix your attention on what it shows.

[　　]