This would further aggravate the congestion on the city roads and also increase the pollution level.

Every vehicle entering the charging zone must pay regardless of its pollution level.

One day the probe surfaces and detects a high pollution level and signals the aliens.

In the winter, one can spot some migratory birds around the lake, though the numbers are dwindling because of the rise in pollution level.

Vehicles entering the charging zone incur a charge of regardless of their pollution level.