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$(A)$ 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 $\left(T_{1 / 2}\right)$ is not constant.
Choose the most appropriate answer from the options given below
$(A)$ Electric monopoles do not exist whereas magnetic monopoles exist.
$(B)$ Magnetic field lines due to a solenoid at its ends and outside cannot be completely
straight and confined.
$(C)$ Magnetic field lines are completely confined within a toroid.
$(D)$ Magnetic field lines inside a bar magnet are not parallel.
$(E)$ $\chi=-1$ is the condition for a perfect diamagnetic material, where $\chi$ is its
magnetic susceptibility.
Choose the correct answer from the options given below: