Chain Reaction

Chain Reaction is the 18th level of Bloons.

Completion
It is fairly easy, as all the player has to do is to hit one of the three Tile Blocks, then the Tack Bloon behind it, which then pops the Tack Bloon next to it and so forth, causing the 'chain reaction' which will allow you to win.

Trivia

 * A chain reaction will pop all of the bloons, hence the level's name.