Following statements related to radioactivity are given below:
(A) Radioactivity is a random and spontaneous process and is dependent on physical and chemical conditions.
(B) The number of un-decayed nuclei in the radioactive sample decays exponentially with time.
(C) Slope of the graph of $\log e$ (no. of undecayed nuclei) Vs. time represents the reciprocal of mean life time ( $\tau$ ).
(D) Product of decay constant ( $\lambda$ ) and half-life time ( $T{1 / 2}$ ) is not constant.
Choose the most appropriate answer from the options given below: