If ${\mu _s}$,${\mu _k}$,and ${\mu _r}$ are coefficients of static friction,sliding (kinetic) friction,and rolling friction respectively,then:

• A
  ${\mu _s} < {\mu _k} < {\mu _r}$
• B
  ${\mu _k} < {\mu _r} < {\mu _s}$
• C
  ${\mu _r} < {\mu _k} < {\mu _s}$
• D
  ${\mu _r} = {\mu _k} = {\mu _s}$