Modify the game so that a new zombie is added to the zombie_group group every 10 seconds or so, using the timer variable. This will ramp up the difficulty and give the player a greater challenge (and...

Modify the game so that a new zombie is added to the zombie_group group every 10 seconds or so, using the timer variable. This will ramp up the difficulty and give the player a greater challenge (and also make it impossible to survive at a certain point, like any good arcade game!).