The velocities of a body executing SHM at displacement ‘a’ and ‘b’ are ‘b’ and ‘a’ respectively. The amplitude of SHM is 

• A

  a + b

• B

  a - b

• C

  $\sqrt{{\mathrm{a}}^2+{\mathrm{b}}^2}$

• D

  $\sqrt{{\mathrm{a}}^2-{\mathrm{b}}^2}$