Choose the most suitable answer A, B, C, or D to complete the following passage.

Air pollution is caused ________(1) ill health in human beings. In a lot of countries, there are ________(2) limiting the amount of smoke factories can produce although there is not enough _______(3) on the effects of smoke on the atmosphere. Doctors have ________(4) that air pollution causes ________ (5) diseases. The gases from the exhausts of cars _______(6) also increased air pollution in most cities. The ________ (7) of petrol produces a poisonous gas, which is often collected in busy streets circled by high buildings. Children _______(8) in areas where there is a lot of lead in the atmosphere can not think as ________(9) as other children, and are clumsy when they _______(10) their hands.