In testing the water supply for various cities for two kinds of impurities commonly found in water, it was found that 40% of cities have water which contains an impurity of type A, 50% have water which contains an impurity of type B, and 20% have water which contains neither one of the two impurities. If a city is chosen at random, what is the pro- bability that its water supply has exactly one impurity of type A or B ?

A) 0.3 B) 0.8 C) 0.1 D) 0.7 E) 0.2