Answer:
A. 1
B. n
C. 0
Step-by-step explanation:
The numbers to the right of the symbols represent the
. Recall that the mass number is the sum of the number of neutrons and the number of protons in an atom's nucleus. The atomic number is the number of protons in an atom's nucleus. For this equation to be true, both sides must agree on the total number of subatomic particles. So, for the mass number:

This means the sum of the protons and neutrons in the particle is 1.
For the atomic number:

This means there are 0 protons in the particle.
The two derived values indicate that the particle is a lone neutron. So, the answer to B is n.