J.J. Thomson's cathode ray tube experiment demonstrated that

• A

  cathode rays are streams of negatively charged ions
• B

  all the mass of an atom is essentially in the nucleus
• Correct answer
  C

  the $e / m$ of electrons is much greater than the $e / m$ of protons
• D

  the $e / m$ ratio of the cathode ray particles changes when a different gas is placed in the discharged tube

Hence, the correct answer is option: 'C'.