Observe the following statements regarding isotones:
a) ${ }^{39}K$  and ${ }^{40}Ca$   are isotones.
b) Nucleides having different atomic number(Z) and mass numbers (a) but same number of neutrons (n) are called isotones.
c) ${ }^{19}F$  and ${ }^{23}Na$  are isotones.
The correct answer is