---Three people got killed by two explosions.---When was __________?---________ was on April 15 __________ the 2013 Boston Marathon was being held.
_______you need any help, please don't hesitate to let me know.
If I had known you were coming, I ______a larger table.
I passed by Joan quickly without greeting her, ______not to see her.
The inventor was determined to go on with the experiment_______countless failures.
To our great relief, the exam turned out to be not so difficult as we_______.