Fifty millimetres of rain across one square kilometre adds up to 50,000 tonnes of water. Such downpours are not rare and on their own they need not cause any significant problem. But if they occur in the wrong place they can cause flash floods, which can be devastating, especially in steep-sided valleys where the torrent can move at tremendous speeds and build up massive momentum. When moving down a steep valley, 15 centimetres of water will sweep a person off their feet; half a metre of water will carry away a car. If water builds up behind debris it can suddenly burst through, creating a wall of water metres high and travelling at speeds up to 45 kilometres per hour.

Question: Where a flash flood might occur would be very difficult or impossible even to predict.
Options:
A. true, B. false, C. uncertain
Answer: "B"