Samples of two different isotopes have different initial activity. The graph below shows

activity$-$versus$-$time for both isotopes. At time $T$ the activity of each sample is the same.

Which sample has a lower number of undecayed nuclei $N$ at time $T?$

$($a$)$ The sample of isotope $A$

$($b$)$ The sample of isotope B

$($c$)$ Both samples have the same number of nuclei