If the nuclear force between two protons,two neutrons,and between a proton and a neutron is denoted by $F_{pp}$,$F_{nn}$,and $F_{pn}$ respectively,then:

Given below are two statements $:$ one is labelled as Assertion $(A)$ and other is labelled as Reason $(R).$
Assertion $(A) :$ Neutrons penetrate matter more readily as compared to protons.
Reason $(R) :$ Neutrons are slightly more massive than protons.

The most stable particle in the Baryon group is

Consider the following statements ($X$ and $Y$ stand for two different elements):
$(I)$ ${ }_{32}^{65} X$ and ${ }_{33}^{65} Y$ are isotopes.
$(II)$ ${ }_{42}^{86} X$ and ${ }_{42}^{85} Y$ are isotopes.
$(III)$ ${ }_{85}^{174} X$ and ${ }_{88}^{177} Y$ have the same number of neutrons.
$(IV)$ ${ }_{92}^{235} X$ and ${ }_{94}^{235} Y$ are isobars.
Which of the above statements are correct?

The mass number of a nucleus is

Which of the following is a fundamental force in nature?