Hydrogen has three isotopes $(A)$,$(B)$ and $(C)$. If the number of neutron$(s)$ in $(A)$,$(B)$ and $(C)$ respectively,are $(x)$,$(y)$ and $(z)$,the sum of $(x)$,$(y)$ and $(z)$ is:

• A
  $4$
• B
  $3$
• C
  $2$
• D
  $1$