When the magnitude of static friction Ff,s is plotted on the vertical axis against an increasing horizontal applied force Fapp for a block that remains at rest, which of the following best describes the portion of the graph before the block begins to slide?
a) A horizontal line at Ff,s=μsFn, independent of Fapp.
b) A curve that rises steeply at first and then gradually levels off to μsFn.
c) A straight line through the origin with slope 1, so that Ff,s=Fapp until a maximum value is reached.
d) A straight line through the origin whose slope equals μs.