$K_2Cr_2O_7$ is preferred over $Na_2Cr_2O_7$ for use in volumetric analysis as a primary standard because

• A
  $Na_2Cr_2O_7$ is hygroscopic while $K_2Cr_2O_7$ is not
• B
  $K_2Cr_2O_7$ is hygroscopic while $Na_2Cr_2O_7$ is not
• C
  $K_2Cr_2O_7$ is pure while $Na_2Cr_2O_7$ is impure
• D
  None of these