_______ the smoke come out of the building, my first thought was to call the fire department as soon as possible.

 _______ the smoke come out of the building, my first thought was to call the fire department as soon as possible.

_______ the smoke come out of the building, my first thought was to call the fire department as soon as possible.

1. A.
   Having seen
2. B.
   When seeing
3. C.
   When I saw
4. D.
   Seeing