Isotopes of the same element must have the same A. atomic number
B. mass number
C. number of nucleons
D. number of neutrons

D. number of neutrons

Explanation:

Isotopes of the same element must have the same number of neutrons. Neutrons of an atom are the uncharged particles within the nucleus.

Isotopy is the existence of two or more atoms of the same element having the same atomic number but having different mass number due to the differences in the number of neutrons in their various nuclei.

Therefore, isotopes differs based on the number of neutrons they contain.

we can conclude that the melting point of table salt is above room temperature (it's actually 801 deg c). the melting point of mercury must be below room temperature (- 39 deg c).

i just learnt this recently but i need to look through my notes again