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 no . of atoms decayed between time $\;t_{1}\;$ & $\;t_{2}\;$ are

Question

$(a)\;\large\frac{R_{1}-R_{2}}{\lambda}\qquad(b)\;\large\frac{R_{2}}{\lambda}\qquad(c)\;\large\frac{R_{1}}{\lambda}\qquad(d)\;None$