Which of the following is best supported by the passage?

33/149

Scientists should be consulted in order to establish uniform limits for all air pollutants.

Human activities have been effective in reducing air pollution.

One of the most important steps in preserving natural lands is to better enforce air pollution laws.

To effectively control pollution, local government should regularly review their air pollution laws.

Giải thích

… in fact the numerical value tells us little until we know how much of an increase this represents over the concentration that would occur naturally in the area

Chọn B