At time $t=0$, a material is composed of two radioactive atoms ${A}$ and ${B}$, where ${N}_{{A}}(0)=2 {N}_{{B}}(0)$ The decay constant of both kind of radioactive atoms is $\lambda$. However, A disintegrates to ${B}$ and ${B}$ disintegrates to ${C}$. Which of the following figures represents the evolution of ${N}_{{B}}({t}) / {N}_{{B}}(0)$ with respect to time $t$ ?

${N}_{{A}}(0)={No} . \text { of } {A} \text { atoms at } {t}=0$

${N}_{{B}}(0)={No} . \text { of } {B} \text { atoms at } {t}=0$

Write and derive the law of radioactivity decay. 

Activity of a radioactive substance can be represented by various unit. Select correct option

The sample of a radioactive substance has $10^6$ nuclei. Its half life is $20 \,s$. The number of nuclei that will be left after $10 \,s$ is nearly …… $\times 10^5$

In a radioactive decay process , the negatively charged emitted $\beta -$ particles are