A radioactive sample contains N N\,N parent nuclei at a given time. The isotope has a half-life T1/2T_{1/2}T1/2. The rate of decay of this sample is proportional to:
NT1/2\dfrac{N}{T_{1/2}}T1/2N
NT1/2N T_{1/2}NT1/2
T1/2N\dfrac{T_{1/2}}{N}NT1/2
N2T1/2N^2 T_{1/2}N2T1/2
