Twelve of the last 16 winners of the school science fair have been seventh grade students. Mona placed 3 red marbles and 1 green marble in a bag to create a simulation to predict whether future science fair winners will be seventh graders. According to Mona's model, what is the probability that the next 4 winners will all be seventh grade students? StartFraction 3 over 16 EndFraction StartFraction 81 over 256 EndFraction Three-fourths StartFraction 7 over 8 EndFraction