A uniform metal chain is placed on a rough table such that one end of chain hangs down over the edge of the table. When one-third of its length hangs over the edge, the chain starts sliding. Then, the coefficient of static friction is

• A

  $\frac{3}{4}$

• B

  $\frac{1}{4}$

• C

  $\frac{2}{3}$

• D

  $\frac{1}{2}$