The decay constant of radio isotope is `lambda`. If `A_(1) and A_(2)` are its activities at times `t_(1) and t_(2)` respectively, the number of nuclei which have decayed during the time `(t_(1)-t_(2))`
A. `A_(1)-A_(2)`
B. `(A_(1)-A_(2))//lambda`
C. `lambda(A_(1)-A_(2))`
D. `(A_(1)t_(1)-A_(2)t_(2)`
