The rate of disintegration of a radioactive element at any time $t$ is proportional to its mass $M$ at that time. If $k (k > 0)$ is the constant of proportionality, find the time required for an initial mass of $1.5 \text{ g}$ to disintegrate to a mass of $0.5 \text{ g}$.

  • A
    $k \log 3$
  • B
    $\frac{1}{k} \log 3$
  • C
    $k \log 5$
  • D
    $\frac{1}{k} \log 5$

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