Answer:
B. 16
Explanation:
Cl -35 and Cl-33 have the same number of protons because it is the same element Cl.
Their masses are different because they have different number of neutrons.
Cl- 33 has 2 neutrons less than Cl-35.
35 - 33 = 2
If Cl-35 has 18 neutrons, then Cl-33 has 18-2=16 neutrons.
