For an exponentially growing microbial culture, the specific growth rate $(\mu)$ is related to its doubling time $\left(\mathrm{t}_{\mathrm{d}}\right)$ by which one of the following relations?
- $\quad t_{d}=\mu$
- $\mathrm{t}_{\mathrm{d}}=\mu^{2}$
- $\mathrm{t}_{\mathrm{d}}=\frac{1}{\mu} \ln 2$
- $\quad t_{d}=\frac{1}{2} \ln \mu$
