Ten litres of the sample was filtered through a membrane filter and the filter was transferred to solid agar media which supports growth of coliform organisms. After incubation at $37{ }^{\circ} \mathrm{C}$ for $48$ hours, the agar plates showed an average of $64$ colonies per plate. What was the average concentration of coliform organisms (in Colony Forming Units per unit volume) in the original water sample?
- $64 \;\mathrm{CFU} / \mathrm{ml}$
- $640 \; \mathrm{CFU} / \mathrm{ml}$
- $6.4 \times 10^{-3} \; \mathrm{CFU} / \mathrm{ml}$
- $64 \times 10^{-3} \; \mathrm{CFU} / \mathrm{ml}$
