Choose the correct option to complete the sentences:

  1. I) A stranger came into the hall _____ he opened the front door.

  2. II) ________ you begin to look at the problem, there is almost nothing you can do about it.

  3. III) ________ extensive inquiries were made at the time, no trace was found of any relative.

  4. IV) You cannot be put on probation _______ you are guilty.