The first of two identical boxes of mass m is sitting on level ground. The second box is sitting on ...

The first of two identical boxes of mass m is sitting on level ground. The second box is sitting on a ramp that makes an angle with the ground. When a force of magnitude F is applied to each box in a direction parallel to the surface it is on, upwards on the box on the ramp, neither box moves. Which statement comparing the friction force on the box on the level, fL, to the friction force on the box on the ramp, fR, is correct?
 a. fR = fL
  b. fR > fL
  c. fR < fL
  d. The coefficient of static friction is needed to determine the correct answer.
  e. Depending on the values of the coefficient of static friction, the angle of elevation of the ramp, the mass of the boxes, and the applied force, answers (a), (b), and (c) are each a possible correct answer.