Two students collect data on three unknown samples in the lab. Their data is presented in the table below.

Element X Element Y Element Z
Number of Protons 19 20 unknown
Number of Neutrons 21 20 unknown
Number of Electrons 19 20
19

Appearance solid, dull gray solid, dull light gray solid, dull gray
Reacts with acid? yes yes yes
Mass Number 40 40 40
Student 1 turned in his lab sheet stating that all three samples must be the same element, because their appearance, reactivity with acid, and mass number were all the same.

Student 2 disagreed and wrote that Element X and Z were the same, while Element Y was different. She argued that the similar appearance, identical number of electrons, and identical mass number means that Element X and Z are the same element.

Which of the following would most disprove the second student's argument?

A. After long-term exposure to air, Element Z also turns light gray


B. New data determines that Element Z has 20 neutrons.


C. A repeat experiment reveals that an atom of the sample of Element Z actually has 20 electrons.


D. A repeat experiment reveals that the sample of Element Z actually has a mass number of 41.