Which of the following interaction is responsible for beta decay?

If the series limit frequency of Balmer series is $v_B$, then the series limit frequency of the Brackett series is

Consider a nucleus ${ }_{30}^{60} \mathrm{X}$. Its approximate density is (take, $1 \mathrm{amu}=1.6 \times 10^{-27} \mathrm{~kg}, R_0=1.2 \times 10^{-15} \mathrm{~m}$ )

As the mass number $A$ increases, which of the following quantities related to a nucleus does not change?