An 80 N box initially at rest is pulled by a horizontal rope on a horizontal table. The coefficients of kinetic and static friction between the box and the table are 0.25 and 0.5 respectively. Find the friction force on the box if the pull is
a) 25 N and the box remains at rest.
b) 25 N and the box moves with uniform velocity.
c) More than 25 N and the box remains at rest.
d) Less than 25 N and the box accelerates.