If activity of a radioactive substance's $\;A_{1}\;$ at time $\;t_{1}\;$ & $\;A_{2}\;$ at time $\;t_{2}\;$ $\;(> t_{1})\;$ . Its decay constant is $\;\lambda\;$ . Then

Question

$(a)\;A_{1}=A_{2}\;e^{\lambda(t_{1}-t_{2})}\qquad(b)\;A_{2}=A_{1}^2\;e^{\lambda(t_{1}-t_{2})}\qquad(c)\;A_{1}=A_{2}^2\;e^{\lambda(t_{1}-t_{2})}\qquad(d)\;A_{2}=A_{1}\;e^{\lambda(t_{1}-t_{2})}$