Which potential solution could a crowded city improve problems with emission levels?
a. encourage citizens to burn wood or methane fires at home
b. allow citizens to use personal automobiles to commute
c. allow mining and manufacturing to operate without regulation
d. encourage citizens to walk or use bikes to travel

d. encourage citizens to walk or use bikes to travel.

Explanation:

The emission levels of the city refers to the daily liberation of gases by the combustion of the fuel probably used for running vehicles.

According to the given situation, the emission levels can only reduce when people use vehicles less.

Therefore, walking and riding bikes can be a good initiative to reduce the emission levels.

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