Final answer:
Thomson observed two glowing dots in a cathode-ray tube with neon gas because the gas contained two different isotopes of neon which had different masses, leading to the creation of two separate glowing spots.
Step-by-step explanation:
The observation of two glowing dots when J. J. Thomson put neon gas into a cathode-ray tube is because the neon was made up of two different isotopes whose atoms had different masses. This is because isotopes of the same element have the same number of protons and therefore the same chemical properties, but different numbers of neutrons which changes their mass.
As the cathode rays passed through the gas, they caused the neon gas to glow, and due to the different masses of the isotopes, the beam was deflected differently, resulting in two separate glowing spots. The correct answer is (C) The neon was made up of two different isotopes whose atoms had different masses.