4 mm/hr
10 mm/hr
20 mm/hr
40 mm/hr
Answer is Right!
Answer is Wrong!
The correct answer is $\boxed{\text{C}}$, 20 mm/hr.
The British Ministry of Health formula for the intensity of rainfall is:
$$I = \dfrac{20}{\sqrt{t}}$$
where $I$ is the intensity of rainfall in mm/hr and $t$ is the time of concentration in minutes.
In this case, $t = 9$ minutes, so the intensity of rainfall is:
$$I = \dfrac{20}{\sqrt{9}} = 20 \sqrt{\dfrac{1}{3}} = \boxed{20 \text{ mm/hr}}$$
The other options are incorrect because they do not correspond to the British Ministry of Health formula.