There are 1000 light bulbs in a row, each one with a switch in front of it. Everytime you pull a switch, its light bulb goes off if it was on, or on if it was off. So, beginning with all the bulbs off, you pull them all, so they are on. Then you pull all the bulbs starting by two, 2 by 2 (so 2,4,6,8...), then the 3's, 4's, 5's up till 1000. In the end, how many bulbs are on, and what numbers?