Puzzle of the Week #160 - Coin Flipping

In this game, the object is to make it so that all the coins are facing heads up, but on each move you must flip three coins together in a line, horizontally or vertically. For instance you could flip the first three coins on the top row, or the last three coins in the second column, or the middle three coins in the fourth row.

As it stands, the game cannot be solved, however if you flip one individual coin from the middle nine before you start, then the game can be solved.

Which of those middle nine coins should you opt to flip?