A horizontal force F is increased slowly on a block initially at rest on a rough table. Which one of the following correctly describes the graph of frictional force f versus applied force F?
a) f is zero until F exceeds μsN, after which f stays constant at μkN.
b) f rises along a line of slope 1 from the origin until F=μsN, after which f drops suddenly to a lower constant value.
c) f rises linearly with F until F=μsN, and then jumps to a higher constant value μkN.
d) f is equal to μsN for all values of F, whether the block moves or not.