Which of the following is true about the coefficient of static friction $(\mu_s)$ and the coefficient of kinetic friction $(\mu_k)$?

• A
  $\mu_s$ is always equal to $\mu_k$
• B
  $\mu_s$ is always greater than $\mu_k$
• C
  $\mu_s$ is always less than $\mu_k$
• D
  Depending upon applications,$\mu_s$ can be greater,less,or equal to $\mu_k$