The graph illustrates how the rate of an enzyme-catalysed reaction varies with substrate concentration. Determine the Michaelis-Menten constant ($K_m$) for this enzyme under these conditions?
- A$0.19\,\text{mmol dm}^{-3}$
- B$0.38\,\text{mmol dm}^{-3}$
- C$1.5\,\text{mmol dm}^{-3}$
- D$5.0\,\text{mmol dm}^{-3}$